[FEATURE] Add YouTubeSearchTool for YouTube Data API v3

Feature Area

Agent capabilities

Is your feature request related to a an existing bug? Please link it here.

NA

Describe the solution you'd like

Add a new YouTubeSearchTool that:

• Searches YouTube videos by keyword using YouTube Data API v3
• Takes search_query (str) and max_results (int, default=5, max=50) as input
• Returns structured data: title, video_id, url, description, published_at
• Uses google-api-python-client for API integration
• Handles API key via YOUTUBE_API_KEY environment variable
• Includes proper error handling and type hints

Describe alternatives you've considered

The existing YouTubeChannelSearchTool and YouTubeVideoSearchTool only perform RAG searches within specific content. They cannot search YouTube by keyword. This new tool fills that gap.

Additional context

I have already implemented this tool in PR #7122 with:

• Production-ready code (430+ lines)
• Comprehensive error handling
• Pydantic validation
• Lazy imports for optional dependencies
• Test coverage

The tool is ready for review and testing.

Willingness to Contribute

Yes, I'd be happy to submit a pull request

Start by reviewing PR #7122, which contains the proposed YouTubeSearchTool implementation, then compare it with the existing YouTubeChannelSearchTool and YouTubeVideoSearchTool. Confirm that keyword search, input limits, structured results, YOUTUBE_API_KEY handling, validation, error handling, lazy imports, type hints, and test coverage meet the issue requirements.