Fix async callable object tools

[stephanlensky]

Makes detection of asynchronous functions more robust in Tool.from_function, allowing usage of asynchronous callable objects, e.g.

class MyAsyncTool:
    def __init__(self):
        self.__name__ = "MyAsyncTool"

    async def __call__(self, x: int) -> int:
        return x * 2

manager = ToolManager()
tool = manager.add_tool(MyAsyncTool())
result = await tool.run({"x": 5})  # 10

Motivation and Context

Fixes #567.

How Has This Been Tested?

Unit tests were added to verify the change.

Breaking Changes

This is a non-breaking change.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context