fix(tests): bind the docker-socket fixtures somewhere sun_path fits

Four more tests in the same family as the /tmp ones this change already
fixes, and they hide for the same reason: the failure depends on how
long $TMPDIR happens to be.

tests/test_shell_routes.py::TestHostDockerAccess (three) and
tests/test_cookbook_docker_access.py::test_container_opt_in_with_unix_
socket_is_allowed each bind an AF_UNIX socket at tmp_path/"docker.sock".
macOS gives sun_path 104 bytes including the terminator. pytest roots
tmp_path at $TMPDIR, which on a stock Mac is a 49-character
/var/folders/<2>/<30>/T/; add pytest-of-<user>/pytest-<n>/ and the
test's own name and the bind path is 115 bytes before the filename.

    OSError: AF_UNIX path too long

Linux allows 108 and roots $TMPDIR at /tmp, so CI never sees it. Under a
shortened $TMPDIR the path lands at exactly 103 and passes — until
pytest's run counter reaches two digits and it becomes 104. That is why
the ledger's counts did not include these: they were measured somewhere
the path fit.

Adds tests/helpers/unix_sockets.bound_unix_socket, which binds under a
short directory and asserts the length before it tries, so the next
socket fixture fails with a sentence rather than an errno. Records the
trap in KNOWN_FAILURES.md along with the instruction to re-measure with
the default $TMPDIR.

tests/KNOWN_FAILURES.md

@@ -6,10 +6,12 @@ uninterpretable: you cannot tell "you broke something" from "you are on a Mac",
 so the usual result is either chasing a non-bug or ignoring a real one.
 
 This is that list. It is a record of observation, not a permission slip: a test
-here is still a test that does not pass, and three of the six below are
-defects someone should fix.
+here is still a test that does not pass, and the three that remain below are
+all still worth someone's time.
 
-Last measured: `lab @ c499c01b` plus the fixes in this change, macOS 15 on Apple Silicon, Python 3.11.
+Last measured: `lab @ c499c01b` plus the fixes in this change, macOS 15 on
+Apple Silicon, Python 3.11, with the **default** `$TMPDIR` — see the socket
+entry below for why that qualifier is load-bearing.
 
 ```
 3 failed, 10658 passed, 6 skipped
@@ -38,9 +40,9 @@ including one holding real data.
 Miss `npm ci` and roughly 36 browser tests fail on `Cannot find package
 'playwright'`. That is not a regression, it is the missing install.
 
-## The three
+## The three that remain, and the seven that no longer do
 
-### Test bugs: fixed
+### Test bugs: comparing an unresolved path against a resolved one
 
 Three failures compared an unresolved `/tmp` path against a resolved
 `/private/tmp` one, and are fixed rather than listed:
@@ -57,6 +59,33 @@ Both now resolve consistently. They are recorded here because the shape recurs:
 on macOS, mixing a resolved and an unresolved temp path is a test bug that
 looks like a platform failure.
 
+### Test bugs: a temp path too long to bind a socket to
+
+Four more, same family, invisible unless `$TMPDIR` is long enough:
+
+- `tests/test_shell_routes.py::TestHostDockerAccess` (three tests)
+- `tests/test_cookbook_docker_access.py::test_container_opt_in_with_unix_socket_is_allowed`
+
+```
+OSError: AF_UNIX path too long
+```
+
+Each bound an `AF_UNIX` socket at `tmp_path / "docker.sock"`. macOS gives
+`sun_path` 104 bytes including the terminator, and pytest's `tmp_path` is
+rooted at `$TMPDIR`, which on a stock Mac is a 49-character
+`/var/folders/<2>/<30>/T/`. Add `pytest-of-<user>/pytest-<n>/` and the test's
+own name and the bind path is 115 bytes before the filename.
+
+This is why the counts above depend on where you run from: under a shortened
+`$TMPDIR` the path lands at 103 and the tests pass, and it tips over the moment
+pytest's run counter reaches two digits. Linux allows 108 bytes and roots
+`$TMPDIR` at `/tmp`, so it never bites there and CI stays green.
+
+They now bind through `tests/helpers/unix_sockets.bound_unix_socket`, which
+puts the socket under a short directory. **Measure with the default `$TMPDIR`**
+— `env -u TMPDIR` or an explicit `/var/folders/...` — or this whole file
+records a run nobody else has.
+
 ### Optional dependency: ffmpeg without a WebP encoder
 
 - `tests/test_inspect_media_tool.py::test_inspect_media_exports_final_decodable_frame_at_exact_duration`

tests/helpers/unix_sockets.py

@@ -0,0 +1,46 @@
+"""Bind an AF_UNIX socket at a path the kernel will actually accept.
+
+``sun_path`` is 104 bytes on macOS, terminator included, so a bind path longer
+than 103 characters fails with ``OSError: AF_UNIX path too long``. pytest's
+``tmp_path`` is rooted at ``$TMPDIR``, which on stock macOS is a 49-character
+``/var/folders/<2>/<30>/T/`` path; adding ``pytest-of-<user>/pytest-<n>/`` and
+the test's own (truncated) name spends the rest of the budget before the
+filename is appended.
+
+That is why this reads as flaky rather than broken. Linux allows 108 bytes and
+roots ``$TMPDIR`` at ``/tmp``, so it never bites there; on macOS whether it
+bites depends on the length of ``$TMPDIR``, the test's name, and how many
+digits pytest's run counter is currently using. A run under a shortened
+``$TMPDIR`` passes, the same checkout under the default one does not.
+
+The path is resolved before it is handed back, for the same reason the rest of
+this change resolves temp paths: on macOS ``/tmp`` is a symlink to
+``/private/tmp``, and a test that binds one spelling and asserts on the other
+is comparing two names for the same socket.
+"""
+
+import os
+import shutil
+import socket
+import tempfile
+from contextlib import contextmanager
+
+# Short enough to leave room for the socket's own name under every platform's
+# sun_path budget. A relative root would depend on the working directory.
+_SHORT_ROOT = os.path.realpath(tempfile.gettempdir() if os.name == "nt" else "/tmp")
+
+
+@contextmanager
+def bound_unix_socket(name="docker.sock"):
+    """Yield the path of a listening AF_UNIX socket, cleaned up on exit."""
+    directory = os.path.realpath(tempfile.mkdtemp(prefix="odysseus-sock-", dir=_SHORT_ROOT))
+    path = os.path.join(directory, name)
+    if len(path) > 103:  # pragma: no cover - guards the guard
+        raise AssertionError(f"socket path is {len(path)} bytes, over the limit: {path}")
+    sock = socket.socket(socket.AF_UNIX)
+    try:
+        sock.bind(path)
+        yield path
+    finally:
+        sock.close()
+        shutil.rmtree(directory, ignore_errors=True)

tests/test_cookbook_docker_access.py

@@ -1,4 +1,3 @@
-import socket
 from unittest.mock import AsyncMock
 
 import pytest
@@ -9,6 +8,7 @@ from starlette.requests import Request
 import routes.cookbook_routes as cookbook_routes
 from routes.cookbook_helpers import ServeRequest, _validate_serve_cmd
 from src.host_docker_access import HOST_DOCKER_ACCESS_HINT
+from tests.helpers.unix_sockets import bound_unix_socket
 
 
 def _model_serve_endpoint():
@@ -57,19 +57,18 @@ async def test_container_cli_only_is_rejected(monkeypatch, tmp_path):
 
 
 @pytest.mark.asyncio
-async def test_container_opt_in_with_unix_socket_is_allowed(monkeypatch, tmp_path):
+async def test_container_opt_in_with_unix_socket_is_allowed(monkeypatch):
     monkeypatch.setattr(cookbook_routes.shutil, "which", lambda binary: "/usr/bin/docker")
-    socket_path = tmp_path / "docker.sock"
 
-    with socket.socket(socket.AF_UNIX) as unix_socket:
-        unix_socket.bind(str(socket_path))
+    # Not tmp_path: binding under $TMPDIR overruns sun_path on macOS.
+    with bound_unix_socket() as socket_path:
         available = await cookbook_routes._binary_available(
             "docker",
             None,
             None,
             in_container=True,
             environ={"ODYSSEUS_ENABLE_HOST_DOCKER": "true"},
-            socket_path=str(socket_path),
+            socket_path=socket_path,
         )
 
     assert available is True

tests/test_shell_routes.py

@@ -5,7 +5,6 @@ import importlib
 import importlib.util
 import json
 import os
-import socket
 import sys
 from pathlib import Path
 from types import SimpleNamespace
@@ -28,6 +27,7 @@ from routes.shell_routes import (
     _venv_activate_prefix,
     DOCKER_IN_CONTAINER_HINT,
 )
+from tests.helpers.unix_sockets import bound_unix_socket
 
 
 def test_shell_routes_import_without_posix_pty_modules(monkeypatch):
@@ -294,30 +294,25 @@ class TestHostDockerAccess:
     def test_socket_without_explicit_opt_in_is_disabled(
         self,
         monkeypatch,
-        tmp_path,
         flag,
     ):
-        socket_path = tmp_path / "docker.sock"
-        with socket.socket(socket.AF_UNIX) as unix_socket:
-            unix_socket.bind(str(socket_path))
+        # Not tmp_path: binding under $TMPDIR overruns sun_path on macOS.
+        with bound_unix_socket() as socket_path:
             if flag is None:
                 monkeypatch.delenv("ODYSSEUS_ENABLE_HOST_DOCKER", raising=False)
             else:
                 monkeypatch.setenv("ODYSSEUS_ENABLE_HOST_DOCKER", flag)
 
-            assert _host_docker_access_enabled(str(socket_path)) is False
+            assert _host_docker_access_enabled(socket_path) is False
 
     def test_explicit_opt_in_with_unix_socket_is_enabled(
         self,
         monkeypatch,
-        tmp_path,
     ):
-        socket_path = tmp_path / "docker.sock"
-        with socket.socket(socket.AF_UNIX) as unix_socket:
-            unix_socket.bind(str(socket_path))
+        with bound_unix_socket() as socket_path:
             monkeypatch.setenv("ODYSSEUS_ENABLE_HOST_DOCKER", "true")
 
-            assert _host_docker_access_enabled(str(socket_path)) is True
+            assert _host_docker_access_enabled(socket_path) is True
 
 
 class TestPackageProbeStatus: