fix: thread-safe singleton pattern in MCPManager

[Jah-yee]

Description

Fixes issue #812 - Thread unsafe singleton pattern in MCPManager

The MCPManager class uses a singleton pattern that is not thread-safe. In a multi-threaded environment, two threads can simultaneously evaluate cls._instance is None as True, resulting in multiple instances being created.

Solution

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

class MCPManager:
    _instance = None
    _lock = threading.Lock()

    def __new__(cls, *args, **kwargs):
        with cls._lock:
            if cls._instance is None:
                cls._instance = super(MCPManager, cls).__new__(cls, *args, **kwargs)
        return cls._instance

This prevents race conditions when multiple threads try to initialize MCPManager concurrently (e.g., when serving multiple users via Gradio WebUI or ASGI servers).

Testing

The fix ensures that only one instance of MCPManager is created even when accessed concurrently from multiple threads.