API compatibility restoration for Gemini, AssemblyAI, and Russian STT

This release restores legacy Gemini TTS and AssemblyAI STT model IDs and the Russian language hint on OpenAI transcription models.

Compatibility support maps gemini:gemini-3.1-flash-tts-preview to google-tts:gemini-3.1-flash-tts-preview across model bodies, allow and deny headers, and API-key routing chains.

The catalog publishes assemblyai:universal-3-5-pro while the legacy assemblyai:u3-rt-pro and assemblyai:universal-3.5-pro pins remain accepted.

The restored multipart path forwards language=ru to OpenAI gpt-4o-transcribe and gpt-4o-mini-transcribe.

For buffered STT requests, a multipart body language field now overrides X-Speko-Language for every supported language. An empty or otherwise invalid body language value returns HTTP 400 even when the header contains a valid value.

The Russian STT constraint stays inside compatible OpenAI transcription models. Unsupported TTS, LLM, and realtime STT routes do not fall back to English.

Action: No migration is required. New integrations should use google-tts:gemini-3.1-flash-tts-preview and assemblyai:universal-3-5-pro; existing integrations can keep the legacy IDs.