`RuntimeError: Attempted to exit cancel scope in a different task` when cleaning up multiple MCPClient instances out-of-order

Describe the bug
If two MCPClient objects are instantiated and cleaned up in non-FILO order (i.e., the first-created client is cleaned up before the second), teardown fails with a cascade of RuntimeError/CancelledError exceptions coming from anyio and mcp.client.stdio.

To Reproduce
Minimal repro:

import os, asyncio, json
from typing import Optional
from contextlib import AsyncExitStack
from mcp import ClientSession, StdioServerParameters
from mcp.types import TextContent
from mcp.client.stdio import stdio_client

class MCPClient:
    def __init__(self, command: str, args: list[str], env: Optional[dict] = None):
        self.session: Optional[ClientSession] = None
        self.command, self.args, self.env = command, args, env
        self._cleanup_lock = asyncio.Lock()
        self.exit_stack: Optional[AsyncExitStack] = None

    async def connect_to_server(self):
        await self.cleanup()
        self.exit_stack = AsyncExitStack()

        server_params = StdioServerParameters(
            command=self.command, args=self.args, env=self.env
        )
        stdio_transport = await self.exit_stack.enter_async_context(stdio_client(server_params))
        self.stdio, self.write = stdio_transport
        self.session = await self.exit_stack.enter_async_context(
            ClientSession(self.stdio, self.write)
        )
        await self.session.initialize()

    async def cleanup(self):
        if self.exit_stack:
            async with self._cleanup_lock:
                await self.exit_stack.aclose()
                self.session = None
            self.exit_stack = None

async def main():
    cfg = {
        "command": "npx",
        "args": ["-y", "@adenot/mcp-google-search"],
        "env": {
            "GOOGLE_API_KEY": os.environ["GOOGLE_API_KEY"],
            "GOOGLE_SEARCH_ENGINE_ID": os.environ["GOOGLE_SEARCH_ENGINE_ID"],
        },
    }

    c1, c2 = MCPClient(**cfg), MCPClient(**cfg)
    await c1.connect_to_server()
    await c2.connect_to_server()

    # Works (FILO)
    # await c2.cleanup()
    # await c1.cleanup()

    # Fails (FIFO)
    await c1.cleanup()      # <-- boom
    await c2.cleanup()

if __name__ == "__main__":
    asyncio.run(main())

Expected behavior

cleanup() should succeed regardless of the order in which multiple MCPClient instances are closed, as long as each instance’s own exit_stack is intact. A single client ought to manage its own lifetime without depending on external FILO discipline.

Actual Traceback

RuntimeError: Attempted to exit cancel scope in a different task than it was entered in
...
asyncio.exceptions.CancelledError: Cancelled by cancel scope ...
...
RuntimeError: Attempted to exit a cancel scope that isn't the current task's current cancel scope

Environment

先執行 issue 中的最小重現，接著檢查 MCPClient.cleanup()、AsyncExitStack、stdio_client 以及 ClientSession teardown 的行為。當獨立建立的用戶端可以依 FIFO 順序進行清理而不發生 RuntimeError 或 CancelledError，且 FILO 順序的清理仍能正常運作時，修正就完成了。