Gate finding (2c, Low): PTB dispatches updates sequentially (no
concurrent_updates) and counts a dispatch when the handler starts, so one
slow-but-bounded handler (sticker vision_analyze at 120s+, an awaited
slash-confirm callback) plus a queued update looked like "no dispatch
progress" after two 90s heartbeats and triggered a fatal rebuild that drops
already-acknowledged updates.
Raise _INGRESS_DISPATCH_STALL_HEARTBEATS to 4 (360s) so the window exceeds
_POLLING_ERROR_TASK_STUCK_TIMEOUT (300s) with margin. Deliberately not gated
on _inflight_update_ids: that would hide a handler that is itself wedged.
Tests drive heartbeats by count; counts updated, semantics unchanged, plus a
270s-no-escalation assertion.
1809f4fec3