Use AwareDateTime when generating field from DateTime(timezone=True) (#407)

[DYNOSuprovo]

Closes #407.

Summary

When generating a marshmallow field for a SQLAlchemy column of type DateTime(timezone=True) (or dialect equivalents such as postgresql.TIMESTAMP(timezone=True), mysql.DATETIME(timezone=True), and mssql.DATETIMEOFFSET), ModelConverter currently generates a default fields.DateTime. This forces schemas to explicitly override auto-generated fields with fields.AwareDateTime.

Solution

• Added _datetime_field_factory and registered sa.DateTime and mysql.DATETIME in ModelConverter.SQLA_TYPE_MAPPING.
• If getattr(data_type, "timezone", False) is True (or if it is an instance of mssql.DATETIMEOFFSET), the factory returns fields.AwareDateTime.
• Otherwise, it falls back to converter.type_mapping.get(dt.datetime, fields.DateTime), fully preserving existing behavior and custom schema TYPE_MAPPING overrides for naive datetimes.
• Added unit tests in tests/test_conversion.py for standard and dialect types (sa.DateTime, postgresql.TIMESTAMP, mysql.DATETIME, mssql.DATETIMEOFFSET, sa.TIMESTAMP) and in tests/test_sqlalchemy_schema.py for SQLAlchemyAutoSchema.

[lafrech]

Looks good.

Please see our AI policy if you haven't already.

Also wondering if we should use a NaiveDateTime in other cases. Let's keep the conversation in #407.

[DYNOSuprovo]

Thank you @lafrech! I will follow up in #407 regarding the NaiveDateTime discussion.