Pydantic serialization issues around timestamp

[cleemullins]

When running the Unit Test suite, ~100 warnings are emitted around datetime serialization.

All the warnings have this same format:

tests/hosting_core/storage/test_file_transcript_storage.py::test_delete_transcript_removes_file
  D:\git\Agents-for-python\venv\Lib\site-packages\pydantic\main.py:528: UserWarning: Pydantic serializer warnings:
    PydanticSerializationUnexpectedValue(Expected `datetime` - serialized value may not be as expected [field_name='timestamp', input_value='2025-10-13T21:52:01.399099+00:00', input_type=str])
    return self.__pydantic_serializer__.to_json(

This Issue is part of my current efforts to cleanup all the broken windows so we can enable strong linting and better build time tests.