Closes #6252

fastapi_mcp has no cycle detection in resolve_schema_references(). Any self-referencing Pydantic model in the OpenAPI schema (compound filters, nested FilterExpression, etc.) causes a RecursionError that takes down the whole MCP server. It also spits out $ref/$defs-heavy schemas that LLMs can't work with, and generates tool names from HTTP verbs (post_get_online_features).

This swaps fastapi_mcp for the MCP Python SDK (FastMCP). Tools are registered with @mcp.tool() and call FeatureStore directly, no internal HTTP round-trip.

What changed

• mcp_server.py -- replaced FastApiMCP wrapper with a create_mcp_server() function that registers tools explicitly
• New registry tools: list_feature_views, list_entities, list_feature_services, list_data_sources
• New data access tools: get_online_features, search_documents
• New materialization tools: materialize, materialize_incremental
• SSE and streamable HTTP transports via mcp_transport config
• Configurable mcp_base_path (default /mcp)
• CORS middleware exposing Mcp-Session-Id for HTTP transport
• Session manager lifecycle tied to FastAPI lifespan
• Elasticsearch timestamp parsing now handles microseconds
• Unit and integration tests rewritten for the new SDK