What changes were proposed in this pull request?

1. Updated AuditDTO to use datetime instead of str for _create_time and _last_modified_time.
2. Configured custom decoder and encoder functions in dataclasses_json metadata to properly parse the ISO-8601 strings into datetime objects.
3. Updated the return types of create_time() and last_modified_time() methods to properly return datetime.
4. Modified python unit tests that incorrectly tested for strings to compare against datetime objects.

Why are the changes needed?

The python client's AuditDTO was failing to deserialize datetime fields from JSON, keeping them as raw ISO-8601 strings (and marked with TODO: Can't deserialized datetime from JSON). This violated the Audit interface, which defines create_time() and last_modified_time() to return datetime objects. This change fixes the type mismatch at runtime.

Fix: #12196

Does this PR introduce any user-facing change?

Yes, AuditDTO.create_time() and AuditDTO.last_modified_time() in the Python client will now correctly return datetime objects instead of strings, aligning with the expected interface.

How was this patch tested?

• Ran and updated existing python unit tests (test_relational_table.py, test_table_dto.py, test_responses.py) which now correctly assert against datetime objects.