Detect supervisor-subprocess trigger count mismatch in Triggerer

[JH0917]

In rare cases the Triggerer subprocess can lose track of a trigger — it
is assigned in the supervisor's running_triggers but no coroutine is
actually running. The affected task instance stays in deferred permanently. A similar symptom was reported in #63913.

Fix: The TriggerRunner now reports num_running (its actual
trigger count) in every TriggerStateChanges message. The supervisor
compares this against len(running_triggers) after finished triggers
are removed but before new ones are added. If they diverge, the
supervisor shuts down so the orchestrator can restart it and recover the
stuck tasks.

related: #63913

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 4.6)

Generated-by: Claude Code (Opus 4.6) following the guidelines

[boring-cyborg]

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