fix(stt): dispatch live-API-only Gemini models over the Live WebSocket transport (#4006)

Addresses #4006 by letting Gemini STT models that only exist on the Live
API transcribe instead of failing.

transcribeGemini sends audio to :generateContent and that is the only Gemini
path. A model that is realtime-only (exposed by the Live API's
bidiGenerateContent WebSocket) therefore fails outright, even though the account
can transcribe it.

open-sse/handlers/geminiLiveStt.js owns the WebSocket lifecycle: opens
:bidiGenerateContent, sends setup frame, waits for setupComplete, streams
audio as realtimeInput media chunks, and settles on turnComplete.
Dispatch is driven by transport marker 'gemini-live'. Adds custom model transport
persistence and selection on the dashboard.