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

[abdelhadi703]

Summary

Add thread-safety to the MCPManager singleton pattern.

Problem: The singleton used __new__ without a mutex lock. In multi-threaded environments, 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.

Fix: Add threading.Lock() as a class variable and wrap the instance creation in a with cls._lock: block.

Changes

• qwen_agent/tools/mcp_manager.py:
  • Add _lock = threading.Lock() class variable
  • Wrap singleton creation in with cls._lock:

Security/Reliability

• Prevents race conditions in multi-threaded environments
• No new dependencies — threading is part of Python stdlib
• threading is already imported in the file

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Contribution by abdelhadisalmaoui0909@outlook.fr