Replace inline `trio.fail_after` + manual `signal.alarm` guard with the
`_testing.trace` CM helpers that auto-capture a full ptree/wchan/py-spy
diag snapshot to disk on timeout.
Deats,
- inner guard: `trio.fail_after` → `fail_after_w_trace` (async CM,
captures on `TooSlowError`).
- outer AFK guard: raw `signal.alarm` → `afk_alarm_w_trace` (sync
CM, captures on `SIGALRM`), only armed under fork backends.
Extracts `_run_and_match()` helper to keep branching clean.
- bump `fail_after_s` from 4/12 → 8/20 to stop borderline flakes
while diag harness accumulates evidence.
- drop `_DIAG_CAP_S` var + manual signal import (now internal to
`afk_alarm_w_trace`).
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit bd07a95d80)
Gud Boi
c8a77fb92b