Wall-cap `test_stale_entry_is_deleted` via `pytest-timeout`

Add a hard process-level wall-clock bound on a test
known to wedge un-Ctrl-C-ably under an in-dev spawn
backend, so an unattended suite run can't hang
indefinitely.

Deats,
- New `testing` dep: `pytest-timeout>=2.3`.
- `test_stale_entry_is_deleted`:
  `@pytest.mark.timeout(3, method='thread')`. The
  `method='thread'` choice is deliberate —
  `method='signal'` routes via `SIGALRM` which can be
  starved by the same GIL-hostage path that drops
  `SIGINT`, so it'd never actually fire in the
  starvation case.

At timeout, `pytest-timeout` hard-kills the pytest
process itself — that's the intended behavior here;
the alternative is the suite never returning.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

(cherry picked from commit 189f4e3f72e9f1eda5d24bcbab5743f7e35bd913)
(factored: kept pyproject + tests/discovery/test_registrar.py parts of
 "Wall-cap `subint` audit tests via `pytest-timeout`"; dropped
 tests/test_subint_cancellation.py)

pyproject.toml

@@ -79,6 +79,11 @@ testing = [
   # https://docs.pytest.org/en/8.0.x/explanation/goodpractices.html#choosing-a-test-layout-import-rules
   "pytest>=8.3.5",
   "pexpect>=4.9.0,<5",
+  # per-test wall-clock bound (used via
+  # `@pytest.mark.timeout(..., method='thread')` on the
+  # known-hanging `subint`-backend audit tests; see
+  # `ai/conc-anal/subint_*_issue.md`).
+  "pytest-timeout>=2.3",
 ]
 repl = [
   "pyperclip>=1.9.0",

tests/discovery/test_registrar.py

@@ -517,6 +517,22 @@ async def kill_transport(
 
 
 # @pytest.mark.parametrize('use_signal', [False, True])
+#
+# Wall-clock bound via `pytest-timeout` (`method='thread'`).
+# Under `--spawn-backend=subint` this test can wedge in an
+# un-Ctrl-C-able state (abandoned-subint + shared-GIL
+# starvation → signal-wakeup-fd pipe fills → SIGINT silently
+# dropped; see `ai/conc-anal/subint_sigint_starvation_issue.md`).
+# `method='thread'` is specifically required because `signal`-
+# method SIGALRM suffers the same GIL-starvation path and
+# wouldn't fire the Python-level handler.
+# At timeout the plugin hard-kills the pytest process — that's
+# the intended behavior here; the alternative is an unattended
+# suite run that never returns.
+@pytest.mark.timeout(
+    3,  # NOTE should be a 2.1s happy path.
+    method='thread',
+)
 def test_stale_entry_is_deleted(
     debug_mode: bool,
     daemon: subprocess.Popen,