docs: Add a type-safe example for custom tool_use_behavior

[MuhammadHamidRaza]

This PR improves an existing documentation example by making it more type-safe.

The change replaces the generic Any type with TContext for the RunContextWrapper in the custom_tool_handler function. This provides a more accurate and robust example for developers, demonstrating a best practice for using the library's context system.

The improved example highlights:

• Proper use of the generic TContext type.
• Enhanced code clarity and type safety.
• A strong reference for other developers to follow when implementing custom tool handlers.

[seratch]

Thanks for sending this, but TContext is an abtract type representation. So, this change is not relevant for the example code.