Pass Gemini AUDIO/TTS generateContent requests through to Google's native
v1beta endpoint instead of converting to chat, with per-credential fallback
(504 timeout, 502 fetch failure). Accept client keys from Bearer,
x-goog-api-key, or ?key= while forwarding only the configured Gemini
credential upstream. Expose native v1beta model names and rewrites, and add
Gemini 3.1 Flash TTS to the catalogs.
Co-authored-by: Cursor <cursoragent@cursor.com>
