feat: add shared MCP connection manager

[VasuBansal7576]

Summary

• add a process-wide MCPConnectionManager with refcounted acquire/release, reconnect, health checks, and cleanup
• add opt-in ToolRegistry integration that uses the shared pool while preserving the default direct MCPClient path
• add unit tests for reuse, refcounting, concurrent access, cleanup, reconnect, health checks, and registry integration

Testing

• uv run pytest core/tests/test_mcp_connection_manager.py core/tests/test_tool_registry.py
• uv run pytest core/tests

Closes #6348