fix: surface command STT fallback usage

[Mushy-Snugglebites-badonkadonk]

Summary

• Allows command STT providers to fall back to local/faster-whisper when fallback_provider: local (or fallback: local) is configured.
• Keeps stt.local: null safe and resolves the fallback through its own local-model configuration instead of leaking a command-provider model override.
• Preserves the originating provider in structured fallback metadata and surfaces a concise transcript-echo warning across normal, pending-voice, and pending-clarify paths without placing raw command failures or metadata in LLM-visible or durable conversation context.
• Documents the command-provider fallback settings.

Validation

Rebased onto main at 2b618fe7e5c31ea62a167a40b3b72c52a88047b2 and verified with focused coverage for command-provider fallback, explicit-null local config, model-override isolation, fresh/pending transcript echoes, and raw clarify replies:

uv run --frozen --with pytest --with pytest-asyncio pytest -q \
  tests/tools/test_transcription_tools.py::TestTranscribeAudioDispatch::test_command_provider_falls_back_to_local_when_configured \
  tests/tools/test_transcription_tools.py::TestTranscribeAudioDispatch::test_command_provider_local_fallback_accepts_null_local_config \
  tests/tools/test_transcription_tools.py::TestTranscribeAudioDispatch::test_command_provider_model_override_does_not_leak_into_local_fallback \
  tests/tools/test_transcription_tools.py::TestTranscribeAudioDispatch::test_command_provider_returns_original_failure_without_fallback \
  tests/gateway/test_stt_config.py
12 passed

uv run --frozen --with ruff ruff check gateway/run.py tools/transcription_tools.py tests/gateway/test_stt_config.py tests/tools/test_transcription_tools.py
All checks passed!

git diff --check also passes. The full transcription-tools file has one unrelated Apple Silicon expectation failure that reproduces unchanged on the rebased main commit; the focused PR coverage above is green.

AI assistance disclosure

This PR was prepared with AI assistance under human direction and reviewed/tested locally before submission.

[teknium1]

Thanks for addressing a verified gap: current main returns a command-provider failure directly at tools/transcription_tools.py:1703-1711, so the configured local fallback is not currently reachable.

Problems

• The added local_cfg = stt_config.get("local", {}) can be None when config contains stt.local: null; the following .get() then raises. Current native local dispatch uses stt_config.get("local") or {} at tools/transcription_tools.py:1660, following the null-subsection fix in 3a394210.
• fallback_provider / fallback is new user-facing command-provider configuration, but website/docs/user-guide/features/tts.md:546-553 does not document either key.

Suggested changes

• Use the existing or {} guard and add a null-stt.local regression test for this fallback path.
• Add the fallback setting and semantics to the STT command-provider docs.

Automated hermes-sweeper review.

[andrexibiza]

Verified against current main (70db671fac) as part of the Vox Lockin provider-registry lane. Not salvageable within lane 05's scope; no duplicate opened.

Findings:

• Merge conflicts with main in 3 files: gateway/run.py, tests/gateway/test_stt_config.py, tests/tools/test_transcription_tools.py (git merge-tree --write-tree origin/main pr-50564 exit 1, content conflicts). The branch needs a rebase.
• File ownership: the PR touches gateway/run.py and website/docs/user-guide/features/tts.md, which are outside lane 05's provider-registry ownership (gateway media/delivery behavior is lane 07 territory per the Vox Lockin manifest). Its core subject — surfacing command-STT fallback usage in gateway clarify replies — is a gateway-behavior class, not the lazy-install registry class.
• The hermes-sweeper review's two gaps stand and are worth fixing in a rebased revision: the added stt_config.get("local", {}) should use the or {} null-guard (per the local: null / edge: null in config.yaml crashes STT/TTS with 'NoneType' object has no attribute 'get' #47318 pattern at transcription_tools.py:1660), and the new fallback_provider config keys need docs in tts.md.

Recommendation: rebase onto main, apply the two review fixes, and re-request review. Lane 05's class (piper registry parity, probe install-safety, null-config guards) ships separately in #78202.