Refactor datetime handling in FileTranscriptStore to eliminate strings

[cleemullins]

Fixes #165

The File Logger was incorrectly converting datetimes to strings as part of dealing with activty processing. Removed the not-needed conversion (string to datetime) to cleanup the underlying code and tests.

The logged warnings looked like:

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 fix is part of my ongoing effort to cleanup all the broken windows in our build and test system.

[Copilot]

Pull Request Overview

This PR refactors datetime handling in the FileTranscriptStore to eliminate unnecessary string conversions that were causing Pydantic serialization warnings. The fix ensures datetime objects are used consistently instead of converting them to ISO string format unnecessarily.

• Removed the _utc_iso_now() helper function that returned ISO string format
• Updated log_activity() to set timestamps as datetime objects directly
• Fixed test code to use datetime objects instead of ISO strings

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
libraries/microsoft-agents-hosting-core/microsoft_agents/hosting/core/storage/transcript_file_store.py	Removed string conversion helper and updated timestamp assignment to use datetime objects
tests/hosting_core/storage/test_file_transcript_storage.py	Updated test variables to use datetime objects instead of ISO strings

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