feat(stt): honor stt.mistral.base_url, completing endpoint parity

[meltforce]

What does this PR do?

stt.provider: mistral can only ever address api.mistral.ai. _transcribe_mistral builds its client as Mistral(api_key=api_key) with no server_url, so no configuration reaches the endpoint.

Every other STT provider already honors a base URL from its own config block — openai (_resolve_openai_stt_client_config), xai, elevenlabs and deepinfra. Mistral is the exception.

The TTS half is not: #73512 introduced what its description calls class-level base_url parity for TTS providers, and _generate_mistral_tts has read tts.mistral.base_url into the SDK's server_url since. This is the same change for the transcription side, in the shape _transcribe_elevenlabs uses — config block, then env, then module constant.

Unset, no server_url is passed at all and the SDK keeps its own endpoint, so an install that does not configure this behaves exactly as before.

Why it matters. A self-hosted gateway or authenticating reverse proxy in front of Mistral is reachable for synthesis and unreachable for transcription, which makes the provider pair unusable in that deployment — with nothing in the configuration to indicate why. Concretely, this came up routing both directions through Tailscale Aperture, whose HTTP connector answers /v1/audio/speech and /v1/audio/transcriptions identically; TTS could be pointed at it, STT could not.

Related Issue

None open that I could find — searched stt base_url, voxtral, and voxtral endpoint across issues and PRs per CONTRIBUTING § Search First. The closest prior art is #73512 (merged), which this continues.

Type of Change

• ✨ New feature (non-breaking change that adds functionality)

Changes Made

• tools/transcription_tools.py — _transcribe_mistral resolves stt.mistral.base_url → STT_MISTRAL_BASE_URL → MISTRAL_STT_BASE_URL (new module constant, empty by default) and passes it to the SDK as server_url only when set.
• tests/tools/test_transcription_tools.py — three cases in TestTranscribeMistral: config base_url becomes server_url (with trailing-slash normalization), an unconfigured provider passes no server_url at all, and the env variable applies when the config block is silent.
• website/docs/user-guide/features/tts.md — the key in the STT config block, and a sentence in the Mistral provider detail.
• website/docs/reference/environment-variables.md — STT_MISTRAL_BASE_URL.

How to Test

1. python -m pytest tests/tools/test_transcription_tools.py -k "Mistral or mistral" — 9 passed.

2. Regression across the transcription suite: python -m pytest tests/tools/test_transcription_tools.py tests/tools/test_pre_transcription_hook.py tests/tools/test_transcription.py tests/tools/test_stt_language_resolution.py — 101 passed.

3. End to end against a real proxy: set

   stt:
     provider: mistral
     mistral:
       model: voxtral-mini-latest
       base_url: "https://<your-gateway>/v1/connectors/Mistral"

   and send a voice message. Verified this way on agent v0.20.6 against a Tailscale Aperture HTTP connector, which returned the correct transcript; without the change the same configuration reaches api.mistral.ai and the base_url value is ignored.

[Enough1122]

AI code review — automated review for reference; please use your judgment.

Overall: Completes endpoint parity for Mistral STT — stt.mistral.base_url → SDK server_url.

What it does

• tools/transcription_tools.py adds MISTRAL_STT_BASE_URL = os.getenv("STT_MISTRAL_BASE_URL", "") (empty default keeps SDK default) and maps config stt.mistral.base_url to SDK server_url (mirrors tts.mistral.base_url pattern).
• tests/tools/test_transcription_tools.py adds test_config_base_url_becomes_server_url and docs website/docs/... updated with env var + config comment.

Non-blocking notes

• Env var is STT_MISTRAL_BASE_URL but config key is stt.mistral.base_url — mapping precedence is config > env > SDK default; verify code checks config.get("base_url") first then fallback to MISTRAL_STT_BASE_URL, not the reverse, so user config overrides env as documented for TTS.
• Test mocks mistralai module — ensure it asserts server_url kwarg name exactly as SDK expects (server_url vs base_url); current test checks server_url, correct.

Small parity completion; no breaking change when unset.

Non-blocking — please use your judgment.