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

[fede-kamel]

Summary

Adds support for callable class instances and bound methods with beta_tool().

This makes it easier to create tools that need access to instance state or request-scoped context (database sessions, API keys, user context, etc.) without having to wrap everything in closure factories.

Before

def setup_tools(session):
    @beta_tool
    def fetch_product(id: int) -> str:
        return session.get(Product, id)
    return [fetch_product]

tools = setup_tools(db_session)

After

class FetchProduct:
    def __init__(self, session):
        self.session = session

    def __call__(self, id: int) -> str:
        """Fetch a product by ID."""
        return self.session.get(Product, id)

tool = beta_tool(FetchProduct(db_session), name="fetch_product")

Bound methods also work:

tool = beta_tool(my_service.get_weather, name="weather")

Test plan

• Unit tests for callable class instances
• Unit tests for bound methods
• Integration tests with tool_runner
• All existing tests pass

Closes #1087