Wall-cap `subint` audit tests via `pytest-timeout`

Add a hard process-level wall-clock bound on the two
known-hanging subint-backend tests so an unattended
suite run can't wedge indefinitely in either of the
hang classes doc'd in `ai/conc-anal/`.

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 is
  starved by the same GIL-hostage path that drops
  `SIGINT` (see `subint_sigint_starvation_issue.md`),
  so it'd never actually fire in the starvation case.
- `test_subint_non_checkpointing_child`: same
  decorator, same reasoning (defense-in-depth over
  the inner `trio.fail_after(15)`).

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 189f4e3ffc)
(MTF-only portion: kept tests/test_subint_cancellation.py)

tests/test_subint_cancellation.py

@@ -161,6 +161,20 @@ def test_subint_happy_teardown(
         trio.run(partial(_happy_path, reg_addr, deadline))
 
 
+# Wall-clock bound via `pytest-timeout` (`method='thread'`)
+# as defense-in-depth over the inner `trio.fail_after(15)`.
+# Under the orphaned-channel hang class described in
+# `ai/conc-anal/subint_cancel_delivery_hang_issue.md`, SIGINT
+# is still deliverable and this test *should* be unwedgeable
+# by the inner trio timeout — but sibling subint-backend
+# tests in this repo have also exhibited the
+# `subint_sigint_starvation_issue.md` GIL-starvation flavor,
+# so `method='thread'` keeps us safe in case ordering or
+# load shifts the failure mode.
+@pytest.mark.timeout(
+    3,  # NOTE never passes pre-3.14+ subints support.
+    method='thread',
+)
 def test_subint_non_checkpointing_child(
     reg_addr: tuple[str, int|str],
 ) -> None: