refactor(validation): enforce exactly-one-set constraint on model fields in class types.Part
#682
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Add a Pydantic V2 @model_validator to the existing genai.types.Part model to enforce data integrity.
This validator ensures that exactly one of the specified optional fields (e.g., field_a, field_b, field_c) has a value other than None. If zero or more than one field is set during validation, a ValueError is now raised.
This refactoring improves the robustness of the model by explicitly enforcing the intended single-choice rule.