fix(tools): add thread-safe lock to MCPManager singleton

The MCPManager singleton uses __new__ without a mutex lock.
In a multi-threaded environment, two threads can simultaneously
evaluate 'cls._instance is None' before either creates the
instance, resulting in two instances being created and breaking
the singleton pattern.

Add threading.Lock() to ensure thread-safe singleton creation.

Fixes #812

Author: abdelhadi703

qwen_agent/tools/mcp_manager.py

@@ -30,10 +30,12 @@
 
 class MCPManager:
     _instance = None  # Private class variable to store the unique instance
+    _lock = threading.Lock()  # Lock to ensure thread-safe singleton creation
 
     def __new__(cls, *args, **kwargs):
-        if cls._instance is None:
-            cls._instance = super(MCPManager, cls).__new__(cls, *args, **kwargs)
+        with cls._lock:
+            if cls._instance is None:
+                cls._instance = super(MCPManager, cls).__new__(cls, *args, **kwargs)
         return cls._instance
 
     def __init__(self):