feat: add optional You.com search engine integration

[mouse-value-add]

Summary

This PR adds You.com as an optional search engine to the open-webSearch multi-engine MCP server, following the existing engine architecture patterns.

What changed

• New engine implementation: Added youcom search engine in src/engines/youcom/
• API integration: Implements You.com Search API with both keyless and authenticated modes
• Configuration updates: Added youcom to SUPPORTED_SEARCH_ENGINES array
• Runtime registration: Integrated youcom engine into the default search executors
• Name normalization: Added support for 'you', 'you.com', and 'youcom' engine names
• Documentation: Updated tool description to mention You.com support

How it works

Keyless Operation (Default)

• Uses You.com Search API without authentication
• 100 free searches per day per IP
• No setup required - works out of the box

Enhanced Operation (Optional)

• Set YDC_API_KEY environment variable for authenticated access
• Higher rate limits and enhanced features per You.com plan
• Graceful fallback to keyless mode if API key is invalid

Usage Examples

MCP Tool Call

{
  "query": "TypeScript tutorials", 
  "engines": ["youcom", "duckduckgo"],
  "limit": 10
}

CLI Usage

npm run search -- --query "AI frameworks" --engines youcom,brave --limit 15

Environment Setup (Optional)

export YDC_API_KEY=your_api_key_here

Error Handling

• 401 Unauthorized: Clear message about checking YDC_API_KEY
• 429 Rate Limited: Suggests upgrading to API key for higher limits
• Network failures: Graceful degradation with informative errors
• Malformed responses: Validates API response structure

Validation Performed

✅ TypeScript compilation passes without errors
✅ All existing tests continue to pass
✅ You.com engine returns properly formatted SearchResult objects
✅ MCP tool integration works correctly
✅ Engine name normalization handles multiple variants
✅ Both keyless and authenticated modes tested
✅ Error scenarios handled gracefully

Backward Compatibility

• ✅ No changes to existing engine behavior
• ✅ No changes to default configuration
• ✅ youcom engine is opt-in via engines parameter
• ✅ Existing MCP clients continue to work unchanged

Integration Quality

This integration demonstrates the clean extensibility of open-webSearch's architecture. You.com fits naturally as another search provider in the multi-engine ecosystem, providing users with additional choice for web search without compromising the existing functionality.

The implementation follows all established patterns:

• Same SearchResult interface as other engines
• Same error handling patterns
• Same configuration and registration approach
• Same optional/fallback philosophy as the rest of the project

Ready for review and merge! 🚀