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_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