MCP STDIO Performance Issue - Async Context Management Bug

[AlfredoJF]

Describe the bug

STDIO MCP connections restart between requests, causing significant performance degradation (2-5 seconds per request instead of expected 10-50ms). This occurs due to an async context management issue where AsyncExitStack cleanup is attempted from a different async task than where it was created.

To Reproduce

1. Create an ADK agent with STDIO MCP toolset:

   from google.adk.tools.mcp_tool import McpToolset
   from google.adk.tools.mcp_tool.mcp_session_manager import StdioConnectionParams
   from mcp import StdioServerParameters
   
   tools=[McpToolset(
       connection_params=StdioConnectionParams(
           server_params=StdioServerParameters(
               command="node", 
               args=["path/to/mcp-server.js"]
           )
       )
   )]

2. Run the agent and observe logs:

   INFO:google_adk.google.adk.tools.mcp_tool.mcp_session_manager:Cleaning up disconnected session: stdio_session
   WARNING:google_adk.google.adk.tools.mcp_tool.mcp_session_manager:Error during disconnected session cleanup: Attempted to exit cancel scope in a different task than it was entered in
   

Expected behaviour

• MCP sessions should persist between tool calls (10-50ms per request)
• No async context management errors
• Sessions only cleaned up when actually disconnected

Desktop Information

• OS: macOS (affects all platforms)
• Python version: 3.10+
• ADK version: current main branch
• LiteLLM: No
• Model: Any (reproduction doesn't depend on model)

Additional context

Root Cause: src/google/adk/tools/mcp_tool/mcp_session_manager.py:330-338

else:
    # Session is disconnected, clean it up
    logger.info('Cleaning up disconnected session: %s', session_key)
    try:
        await exit_stack.aclose()  # ← Problem: called from wrong async task
    except Exception as e:
        logger.warning('Error during disconnected session cleanup: %s', e)
    finally:
        del self._sessions[session_key]

Problem: AsyncExitStack created during agent initialisation (main task) but cleanup attempted from tool execution task (different async context), violating anyio's cancel scope requirements.

Impact: Affects all STDIO MCP users, especially plugin-based tools and performance-critical applications.

[AlfredoJF]

Potential related issues:
#2979
#2927