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/setup.py

@@ -430,6 +430,8 @@ def _print_setup_summary(config: dict, hermes_home):
         tool_status.append(("Text-to-Speech (MiniMax)", True, None))
     elif tts_provider == "mistral" and get_env_value("MISTRAL_API_KEY"):
         tool_status.append(("Text-to-Speech (Mistral Voxtral)", True, None))
+    elif tts_provider == "gemini" and (get_env_value("GEMINI_API_KEY") or get_env_value("GOOGLE_API_KEY")):
+        tool_status.append(("Text-to-Speech (Google Gemini)", True, None))
     elif tts_provider == "neutts":
         try:
             import importlib.util
@@ -913,6 +915,7 @@ def _setup_tts_provider(config: dict):
         "xai": "xAI TTS",
         "minimax": "MiniMax TTS",
         "mistral": "Mistral Voxtral TTS",
+        "gemini": "Google Gemini TTS",
         "neutts": "NeuTTS",
     }
     current_label = provider_labels.get(current_provider, current_provider)
@@ -935,10 +938,11 @@ def _setup_tts_provider(config: dict):
             "xAI TTS (Grok voices, needs API key)",
             "MiniMax TTS (high quality with voice cloning, needs API key)",
             "Mistral Voxtral TTS (multilingual, native Opus, needs API key)",
+            "Google Gemini TTS (30 prebuilt voices, prompt-controllable, needs API key)",
             "NeuTTS (local on-device, free, ~300MB model download)",
         ]
     )
-    providers.extend(["edge", "elevenlabs", "openai", "xai", "minimax", "mistral", "neutts"])
+    providers.extend(["edge", "elevenlabs", "openai", "xai", "minimax", "mistral", "gemini", "neutts"])
     choices.append(f"Keep current ({current_label})")
     keep_current_idx = len(choices) - 1
     idx = prompt_choice("Select TTS provider:", choices, keep_current_idx)
@@ -1045,6 +1049,19 @@ def _setup_tts_provider(config: dict):
                 print_warning("No API key provided. Falling back to Edge TTS.")
                 selected = "edge"
 
+    elif selected == "gemini":
+        existing = get_env_value("GEMINI_API_KEY") or get_env_value("GOOGLE_API_KEY")
+        if not existing:
+            print()
+            print_info("Get a free API key at https://aistudio.google.com/app/apikey")
+            api_key = prompt("Gemini API key for TTS", password=True)
+            if api_key:
+                save_env_value("GEMINI_API_KEY", api_key)
+                print_success("Gemini TTS API key saved")
+            else:
+                print_warning("No API key provided. Falling back to Edge TTS.")
+                selected = "edge"
+
     # Save the selection
     if "tts" not in config:
         config["tts"] = {}