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/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: