Refactor backend for FastMCP integration

[drernie]

Summary

• add a top-level FastMCP bundle scaffold under mcp_bundle/
• extract a shared Python backend service used by both the legacy stdin backend and the MCP adapter
• add unit tests for the MCP adapter and backend service seam

Testing

• python3 -m unittest discover -s mcp_bundle/tests -p 'test_*.py'
• python3 -c "from mcp_bundle.server.main import _build_server; server = _build_server(); print(type(server).name)"