feat: add comprehensive PyPI search functionality with advanced filtering

- Implemented PyPISearchClient with semantic search, filtering, and sorting
- Added 4 new search tools: search_packages, search_by_category, find_alternatives, get_trending_packages
- Created SearchFilter and SearchSort classes for flexible configuration
- Added SearchError exception for search-specific error handling
- Comprehensive test suite with 13 tests covering all search functionality
- Enhanced MCP server with 4 new search endpoints
- Support for filtering by Python version, license, category, downloads, maintenance status
- Multiple sorting options: relevance, popularity, quality, recency, name, downloads
- Semantic search using description similarity scoring
- Category-based package discovery with intelligent keyword matching
- Package alternatives finder using metadata analysis
- Trending packages analysis with download activity tracking
- Robust fallback mechanisms using curated package database
- All tests passing (13/13)

This implements feature #6 from the roadmap: "Advanced PyPI Search with filtering by Python version, license, maintenance status and sorting by popularity, recency, quality score with semantic search capabilities"

pypi_query_mcp/core/exceptions.py

@@ -54,3 +54,11 @@ class PyPIServerError(PyPIError):
         if not message:
             message = f"PyPI server error (HTTP {status_code})"
         super().__init__(message, status_code=status_code)
+
+
+class SearchError(PyPIError):
+    """Raised when search operations fail."""
+
+    def __init__(self, message: str, query: str | None = None):
+        super().__init__(message)
+        self.query = query