1. Type Annotation Improvements:
   - Changed kwargs: Any to kwargs: Dict[str, Any] in _run_loop method
   - Changed dict[str, Any] to Dict[str, Any] in _execute_event_loop_cycle
   - Improved all Callable type annotations to Callable[..., Any]

2. Default Parameter Value Fix:
   - Changed callback_handler default from PrintingCallbackHandler() to None
   - This avoids the anti-pattern of using a mutable default parameter value

3. Parameter Mutation Protection:
   - Added .copy() to kwargs in both __call__ and stream_async methods
   - This prevents modifications to the kwargs dictionary from affecting the original

This addresses PR feedback by:
1. Adding a sentinel object _DEFAULT_CALLBACK_HANDLER to distinguish between default and explicit None
2. Using the sentinel as the default parameter value for callback_handler
3. Creating a new PrintingCallbackHandler instance when the sentinel is used
4. Preserving the behavior where explicitly passing None uses null_callback_handler
5. Updating the docstring to reflect these changes

…ndler