fix: Add thread-safe singleton pattern for MCPManager

[Jah-yee]

Description

The MCPManager class in qwen_agent/tools/mcp_manager.py uses a singleton pattern that is not thread-safe. In a multi-threaded environment (e.g., Gradio WebUI or ASGI server), two threads can both evaluate cls._instance is None as True simultaneously, resulting in duplicate MCP server connections and inconsistent state.

Fix

This PR adds a threading.Lock to ensure only one instance is created even when multiple threads access __new__ concurrently.

class MCPManager:
    _instance = None
    _lock = threading.Lock()  # Lock for thread-safe singleton pattern

    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

Related Issue

Fixes #812