Each MCP server now connects in its own asyncio.Task to isolate anyio
cancel scopes and prevent 'exit cancel scope in different task' errors
when multiple servers (especially mixed transport types) are configured.
Changes:
- connect_mcp_servers() returns dict[str, AsyncExitStack] instead of None
- Each server runs in separate task via asyncio.gather()
- AgentLoop uses _mcp_stacks dict to track per-server stacks
- Tests updated to handle new API
worenidewen
