feat(voice): add Gemini 2.5 Native TTS Provider

[fav-devs]

🎤 Add Gemini 2.5 Native Voice TTS Provider

This PR implements Google's Gemini 2.5 native text-to-speech capabilities as an official voice provider in VoltAgent.

✨ Features

• 30 High-Quality Voices - Professional voices with distinct characteristics (Zephyr, Puck, Kore, Fenrir, etc.)
• Single-Speaker TTS - Standard text-to-speech with voice selection
• Multi-Speaker Conversations - Support for up to 2 speakers with different voice assignments
• Controllable Speech - Natural language style prompts (e.g., "Say with great excitement")
• 24 Language Support - Automatic language detection and support
• Professional Audio Quality - 24kHz, 16-bit, mono PCM output
• TypeScript Support - Complete type definitions and interfaces
• Consistent Architecture - Follows existing VoltAgent voice provider patterns

📁 Files Added/Modified

Core Implementation

• packages/voice/src/providers/gemini/index.ts - Main GeminiVoiceProvider class
• packages/voice/src/providers/gemini/types.ts - TypeScript definitions and voice metadata
• packages/voice/src/index.ts - Export new provider
• packages/voice/package.json - Added @google/genai dependency

Documentation & Examples

• examples/with-voice-gemini/ - Complete example with usage demonstrations
• website/docs/agents/voice.md - Updated documentation

🚀 Usage Examples

Single-Speaker TTS

const geminiVoice = new GeminiVoiceProvider({
  apiKey: process.env.GEMINI_API_KEY!,
  voice: "Kore", // Firm style
});

const audioStream = await geminiVoice.speak("Hello from Gemini!");

Multi-Speaker Conversations

const audioStream = await geminiVoice.speak(conversationText, {
  speakers: [
    { speaker: "Alice", voice: "Leda" }, // Youthful
    { speaker: "Bob", voice: "Gacrux" },  // Mature
  ],
});

Style-Controlled Speech

const audioStream = await geminiVoice.speak("Welcome!", {
  voice: "Fenrir",
  style: "Say with great excitement and enthusiasm",
});

🎯 Benefits

• Cost Efficiency - Potentially lower costs than dedicated TTS services
• Quality - Professional-grade voice synthesis
• Flexibility - 30 voice options across 24 languages
• Control - Natural language style control
• Integration - Seamless with existing Gemini AI workflows
• Multi-speaker - Built-in conversation support

🔧 Technical Details

• No Breaking Changes - Fully backward compatible
• Consistent API - Implements BaseVoiceProvider interface
• Error Handling - Comprehensive error handling and event emission
• No Fake Streaming - Honest about capabilities (generates complete audio files like OpenAI)
• Dependencies - Added @google/genai ^1.3.0

✅ Testing

• TypeScript compilation successful
• All voice provider interface methods implemented
• Error handling for missing API keys
• Voice metadata and listing functionality
• Example application runs successfully
• Documentation updated

📚 Documentation

• Updated voice provider documentation
• Complete example with README
• Usage patterns and best practices
• Performance notes and limitations

🎉 Ready for Review

This implementation adds Gemini 2.5 native TTS as a first-class voice provider in VoltAgent, giving users access to Google's cutting-edge voice synthesis technology with the same consistent API they're used to.

The provider is production-ready and follows all existing VoltAgent patterns and conventions.

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Implementation Highlights:

1. Voice Variety: 30 professionally crafted voices with unique characteristics
2. Multi-Speaker Support: Create conversations with up to 2 different speakers
3. Style Control: Use natural language to control speech tone and delivery
4. Language Support: Automatic detection across 24 languages
5. Quality: Professional 24kHz audio output
6. Integration: Seamless compatibility with existing VoltAgent architecture

Example Use Cases:

• Podcast generation with multiple speakers
• Audiobook creation with style control
• Multilingual applications
• Voice-enabled AI assistants
• Educational content with engaging narration

This contribution expands VoltAgent's voice capabilities significantly while maintaining the high standards of code quality and user experience that VoltAgent is known for.

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[omeraplak]

Hey, Thank you for this PR. Quick question: Is there any particular reason we updated this line?