feat(assemblyai): warn when audio stops flowing to the WebSocket

[gsharp-aai]

Summary

Complementary to #5476 (recv-side silence warning).

#5476 warns when AssemblyAI sends no messages for 15s+. That's half the diagnostic picture — it can't distinguish "AssemblyAI is stalled" from "the plugin stopped receiving audio from upstream and is correctly silent." When a session appears wedged, both interpretations are on the table and the current logs can't separate them.

This PR adds the symmetric send-side warning: if no audio frames have been written to the WebSocket for 15s+ while the recv loop is also silent, emit AssemblyAI no audio frames sent for Ns session=<id>, re-emitted every 15s while the drought continues.

Together, the two absence-of-activity signals answer the question by elimination:

send-stopped warning	recv-silence warning	Diagnosis
silent	firing	AssemblyAI stalled — frames flowing, no response
firing	firing	Upstream stalled — plugin got no audio
silent	silent	healthy

Implementation

• Adds self._last_frame_sent_at: float | None = None, set on every successful ws.send_bytes(...) in send_task.
• In recv_task, tracks consecutive_timeouts in 5s ticks (matches the cadence feat(assemblyai): log connection lifecycle, silence, and session correlators #5476 introduces on the recv side). Every 3rd tick (15s), check time.time() - _last_frame_sent_at and warn if it exceeds 15s.
• No separate watchdog coroutine. Overhead on the hot path is one time.time() assignment per frame.
• Guarded on _last_frame_sent_at is not None so the warning doesn't fire during the initial connect window before any audio has been sent.

Impact

• Default log level: warning only, and only when the anomaly is real.
• Healthy sessions: silent (by design — mirrors feat(assemblyai): log connection lifecycle, silence, and session correlators #5476's absence-is-the-signal approach).

Relationship to #5476

This PR is self-contained and doesn't depend on #5476 being merged first — it introduces its own consecutive_timeouts counter in recv_task. If #5476 lands first, the counter line in this PR becomes a conflict trivially resolvable by keeping the existing one.

Test plan

• make format lint type-check pass
• Healthy session: no send-stopped warning emitted
• Simulated upstream stall (close input_ch while keeping WS open): send-stopped warning fires at 15s, 30s, 45s
• Simulated AAI stall (mock WS that accepts but never responds): recv-silence warning fires, send-stopped stays silent

[devin-ai-integration]

✅ Devin Review: No Issues Found

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