feat(tools): support callable class instances and bound methods

[majiayu000]

Summary

Add support for using callable class instances and bound methods as tools in beta_tool. This enables a more natural pattern for tools that need access to instance state or context.

Changes

• Added _normalize_callable() helper function that extracts the bound __call__ method from callable class instances
• Modified BaseFunctionTool.__init__ to normalize callables before processing
• Added tests for callable class instances and bound methods

Usage

Callable class instance:

class FetchProduct:
    def __init__(self, ctx: dict):
        self.ctx = ctx
    
    def __call__(self, product_id: int) -> str:
        """Fetch a product by ID."""
        return f"Product {product_id} from {self.ctx['session']}"

instance = FetchProduct({"session": "test"})
tool = beta_tool(instance, name="fetch_product")

Bound method:

class WeatherService:
    def __init__(self, api_key: str):
        self.api_key = api_key
    
    def get_weather(self, location: str) -> str:
        """Get weather for a location."""
        return f"Weather in {location}"

service = WeatherService("secret-key")
tool = beta_tool(service.get_weather, name="weather")

Closes #1087

Test plan

• Added unit tests for callable class instances
• Added unit tests for bound methods
• Verified existing function tests still pass
• Ran ./scripts/lint - all checks pass
• Ran ./scripts/test tests/lib/tools/test_functions.py - 23 tests pass