5.0.8
(fix): Eliminate Pydantic v2 deprecation warnings in UniversalBaseModel. Under Pydantic v2,
the class-based Config pattern with smart_union and allow_population_by_field_name
emits deprecation warnings. UniversalBaseModel now uses model_config: ConfigDict with
populate_by_name=True, validate_by_name=True, and validate_by_alias=True (for v2.11+
support) for v2, and a custom @model_serializer for datetime serialization instead of
the deprecated json_encoders. Pydantic v1 continues to use the original class-based Config.
