feat(stt): unify language resolution across all STT providers

Class-level fix for the 'STT transcribes the wrong language' issue family
(#55551, #50181 and siblings). Previously language handling was per-provider
chaos: local honoured stt.local.language, Groq/OpenAI/Mistral/DeepInfra sent
no language hint at all, xAI silently forced 'en', ElevenLabs used its own
language_code key, and there was no global setting.

- New _resolve_stt_language() helper: stt.<provider>.language >
  stt.language (new global key) > HERMES_LOCAL_STT_LANGUAGE > auto-detect.
- Threaded through ALL providers: local, local_command, groq, openai,
  mistral, xai, elevenlabs, deepinfra (shared OpenAI handler), command
  providers, and plugin dispatch.
- xAI no longer forces English when nothing is configured (auto-detect).
- Mistral Voxtral now receives a language hint when configured.
- stt.groq.model is now honoured from config (previously env-only).
- DEFAULT_CONFIG gains stt.language, stt.groq, stt.xai, stt.mistral.language.
- Tests: tests/tools/test_stt_language_resolution.py (11 tests, sabotage-
  verified) + full transcription suite green (236 passed).

Builds on cherry-picked contributor work from #19786 (@zombopanda),
#23161 (@materemias), #50684 (@BlackishGreen33).

cli-config.yaml.example

@@ -1128,8 +1128,10 @@ stt:
     model: "base"              # tiny | base | small | medium | large-v3 | turbo
     # language: ""             # auto-detect; set to "en", "es", "fr", etc. to force
     # initial_prompt: ""       # Optional faster-whisper prompt, e.g. bias Chinese output to simplified Chinese
+  # language: ""               # GLOBAL language hint for every STT provider (per-provider language wins)
   # groq:
-  #   language: ""             # blank = use HERMES_LOCAL_STT_LANGUAGE if set, else auto-detect
+  #   model: "whisper-large-v3-turbo"
+  #   language: ""             # blank = stt.language > HERMES_LOCAL_STT_LANGUAGE > auto-detect
   openai:
     model: "whisper-1"         # whisper-1 | gpt-4o-mini-transcribe | gpt-4o-transcribe
     language: ""               # auto-detect; set to "en", "es", "fr", etc. to force