Feat use native anthropic format api

[BIGPPWONG]

Summary

• Add Anthropic passthrough support to enable direct integration with Anthropic-compatible providers (e.g., LiteLLM)
• Implement native Anthropic API format support with full SSE streaming capabilities
• Maintain backward compatibility with existing OpenAI format routing

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🚀 Features Added

Anthropic Passthrough Support

✅ Native Anthropic Format: Direct passthrough of original Anthropic API requests
✅ SSE Streaming: Full Server-Sent Events streaming support for real-time responses
✅ Smart URL Handling: Intelligent API endpoint construction (both basic and full URLs)
✅ Auto-detection: Automatic streaming/non-streaming response handling
✅ Backward Compatibility: Existing configurations continue to work unchanged

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Technical Implementation

• New anthropicPassthrough middleware: src/middleware/anthropicPassthrough.ts:1
• Enhanced router type detection: src/utils/router.ts:1
• Updated provider type constants: src/constants.ts:1
• Integrated into main request pipeline: src/index.ts:1
• Comprehensive documentation updates

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📝 Configuration

Provider Configuration

Add type field to enable Anthropic passthrough:

{
  "Providers": [
    {
      "name": "anthropic-direct",
      "type": "anthropic",
      "api_base_url": "https://api.anthropic.com",
      "api_key": "your-api-key",
      "models": ["claude-3-5-sonnet-20241022"]
    }
  ]
}

URL Configuration Options

• Basic URL: "https://api.anthropic.com" → auto-constructs to https://api.anthropic.com/v1/messages
• Full URL: "https://your-proxy.com/v1/messages" → uses as-is

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🔄 Backward Compatibility

• Existing providers without type field default to "openai" format
• No breaking changes to current configurations
• Gradual migration path available

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🧪 Testing

• Configurations validated with both Anthropic official API and proxy services
• SSE streaming tested with real-time response scenarios
• Error handling and resource cleanup verified

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📚 Documentation

• Updated README.md and README_zh.md with comprehensive configuration guides
• Included usage examples and troubleshooting tips