test(git-http): wire the resolver into the access-hook-503 regression test

b1850be's fail-closed-503 test (rebased in from main) built a git-http
server with a flat repo.git and no policy_resolver. The resolver-only
data plane on this branch denies an unattributed request with 404 before
it reaches the access-hook path the test exercises, so it saw 404 != 503.

Nest the bare repo under <root>/<_BID>/ and set _FixedResolver(_BID) on
the server, matching every other test in this module, so the request is
attributed and reaches the access-hook (mocked to raise PermissionError)
that the 503 fail-closed behavior guards.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UoEZHDjv84ChoZbozQERhJ

tests/unit/test_git_http_backend.py

@@ -387,12 +387,13 @@ class TestGitHttpBackend(unittest.TestCase):
 
         with tempfile.TemporaryDirectory() as tmp:
             root = Path(tmp)
-            (root / "repo.git").mkdir()
+            (root / _BID / "repo.git").mkdir(parents=True)
             old_root = os.environ.get("GIT_PROJECT_ROOT")
             os.environ["GIT_PROJECT_ROOT"] = str(root)
             self.addCleanup(self._restore_env, old_root)
 
             server = ThreadingHTTPServer(("127.0.0.1", 0), GitHttpHandler)
+            server.policy_resolver = _FixedResolver(_BID)  # type: ignore[attr-defined]
             thread = threading.Thread(target=server.serve_forever, daemon=True)
             thread.start()
             self.addCleanup(server.shutdown)