fix: reap zombie subprocesses in web_server action status and meet_bot cleanup

- web_server.py: after proc.poll() returns a non-None exit code, call
  proc.wait() to reap the child and move the entry from _ACTION_PROCS
  to _ACTION_RESULTS. Previously .poll() alone left <defunct> zombies.
- meet_bot.py: terminate and wait on the pcm_pump subprocess (paplay/
  ffmpeg) during the finally-block teardown. Previously leaked on every
  normal bot exit.
- tests: add test_action_status_reaps_completed_process and
  test_action_status_ignores_wait_failure covering both the happy path
  and the wait()-raises-OSError edge case.

Closes #38032

plugins/google_meet/meet_bot.py

@@ -699,7 +699,13 @@ def run_bot() -> int:  # noqa: C901 — orchestration, explicit branches
 
             context.close()
             browser.close()
-            # v2: teardown realtime speaker + audio bridge.
+            # v2: teardown PCM pump, speaker thread, and audio bridge.
+            if rt.get("pcm_pump"):
+                try:
+                    rt["pcm_pump"].terminate()
+                    rt["pcm_pump"].wait(timeout=3)
+                except Exception:
+                    pass
             if rt["speaker_stop"]:
                 try:
                     rt["speaker_stop"]()