Add type validation to prompt_with_handoff_instructions function

[AbdulSamad94]

Summary

This PR adds runtime type validation to the prompt_with_handoff_instructions function to ensure it only accepts string inputs. Previously, the function had a type hint but no runtime validation, so passing non-string types did not raise any error, which could lead to unexpected behavior or silent failures.

Changes

• Added isinstance(prompt, str) check in prompt_with_handoff_instructions
• Raises TypeError with descriptive message when non-string types are provided
• Updated function docstring to document the new validation and exception

Testing

The validation has been tested with various input types:

• Valid strings: Work as expected
• Invalid types (int, list, None, etc.): Previously caused silent failures, now properly raise TypeError with a clear message

[seratch]

Thanks for sending this PR. However, we prefer suggesting to use mypy or similar solution to detect this type of coding error.