fix: replace threading.Lock with anyio.Lock for Ray deployment compatibility

- Replace threading.Lock() with anyio.Lock() in StreamableHTTPSessionManager
- This resolves serialization issues when deploying StreamableHTTP services with Ray
- threading.Lock objects cannot be pickled/serialized, causing deployment failures
- anyio.Lock provides equivalent functionality while being fully serializable
- Update lock usage from 'with' to 'async with' for proper async context

Author: only-weng

src/mcp/server/streamable_http_manager.py

@@ -4,7 +4,6 @@
 
 import contextlib
 import logging
-import threading
 from collections.abc import AsyncIterator
 from http import HTTPStatus
 from typing import Any
@@ -75,7 +74,7 @@ def __init__(
         # The task group will be set during lifespan
         self._task_group = None
         # Thread-safe tracking of run() calls
-        self._run_lock = threading.Lock()
+        self._run_lock = anyio.Lock()
         self._has_started = False
 
     @contextlib.asynccontextmanager
@@ -97,7 +96,7 @@ async def lifespan(app: Starlette) -> AsyncIterator[None]:
                 yield
         """
         # Thread-safe check to ensure run() is only called once
-        with self._run_lock:
+        async with self._run_lock:
             if self._has_started:
                 raise RuntimeError(
                     "StreamableHTTPSessionManager .run() can only be called "