feat(tts): add Google Gemini TTS provider (#11229)

Adds Google Gemini TTS as the seventh voice provider, with 30 prebuilt
voices (Zephyr, Puck, Kore, Enceladus, Gacrux, etc.) and natural-language
prompt control. Integrates through the existing provider chain:

- tools/tts_tool.py: new _generate_gemini_tts() calls the
  generativelanguage REST endpoint with responseModalities=[AUDIO],
  wraps the returned 24kHz mono 16-bit PCM (L16) in a WAV RIFF header,
  then ffmpeg-converts to MP3 or Opus depending on output extension.
  For .ogg output, libopus is forced explicitly so Telegram voice
  bubbles get Opus (ffmpeg defaults to Vorbis for .ogg).
- hermes_cli/tools_config.py: exposes 'Google Gemini TTS' as a provider
  option in the curses-based 'hermes tools' UI.
- hermes_cli/setup.py: adds gemini to the setup wizard picker, tool
  status display, and API key prompt branch (accepts existing
  GEMINI_API_KEY or GOOGLE_API_KEY, falls back to Edge if neither set).
- tests/tools/test_tts_gemini.py: 15 unit tests covering WAV header
  wrap correctness, env var fallback (GEMINI/GOOGLE), voice/model
  overrides, snake_case vs camelCase inlineData handling, HTTP error
  surfacing, and empty-audio edge cases.
- docs: TTS features page updated to list seven providers with the new
  gemini config block and ffmpeg notes.

Live-tested against api key against gemini-2.5-flash-preview-tts: .wav,
.mp3, and Telegram-compatible .ogg (Opus codec) all produce valid
playable audio.

hermes_cli/tools_config.py

@@ -172,6 +172,15 @@ TOOL_CATEGORIES = {
                 ],
                 "tts_provider": "mistral",
             },
+            {
+                "name": "Google Gemini TTS",
+                "badge": "preview",
+                "tag": "30 prebuilt voices, controllable via prompts",
+                "env_vars": [
+                    {"key": "GEMINI_API_KEY", "prompt": "Gemini API key", "url": "https://aistudio.google.com/app/apikey"},
+                ],
+                "tts_provider": "gemini",
+            },
         ],
     },
     "web": {