Improve actor abort/timeout handling, add TTS request IDs and VAD error events

[abdufelsayed]

Motivation

• Prevent duplicate/late error emissions and race conditions when actors time out or are aborted by ensuring settled/aborted state is tracked.
• Propagate VAD errors through the machine and metrics so VAD failures are reported like other providers.
• Correlate TTS chunks/completion/errors to the originating request so stale audio from previous/filler TTS requests can be ignored.

Description

• Add settled/timeout-safe error handling to llm and tts actors by introducing isSettled, timeoutId, and a finishWithError helper and by clearing timeouts on completion.
• Include requestId on TTS actor input and on internal _tts:* events and wire the tts actor to attach requestId to chunk, complete, and error events.
• Add activeTTSRequestId to the machine context and implement guards/actions (matchesActiveTTSRequest, speakingCurrentTTS, hasPendingSentencesForCurrentTTS) to ensure only the active TTS request affects state transitions and audio streaming.
• Make vad and turn-detector actors and stt actor return early if abortSignal is already aborted to avoid starting providers after cancellation.
• Add _vad:error event type and include VAD errors in the machine's recordError/emitAgentError flows and metrics.
• Adjust audio streamer to use ArrayBuffer(0) for empty payloads and ensure TTS spawn flows generate nanoid() request ids for systemId and activeTTSRequestId tracking.
• Update fake provider test helpers to support emitError on the VAD fake provider.

Testing

• Ran the unit tests (Vitest) including updated files test/agent/error-recovery.test.ts and test/agent/interruption.test.ts, which exercise VAD error propagation, LLM/TTS timeout deduplication, and ignoring stale filler audio; all tests passed.
• Verified new event typing for _tts:* and _vad:error via the test suite; no type regressions were reported.
• Confirmed timeouts clear timers and do not produce additional agent:error events after a late provider error in the tests.

---

Codex Task