feat: add Pydantic BaseModel serialization support

[PondererOfEnigmas]

Add Pydantic BaseModel serialization support

Previously, ObjectModel fields containing BaseModel instances would trigger a "Object is not JSON serializable" error, when trying to use the save method.
This PR adds native support for serializing Pydantic BaseModel instances in _prepare_value function

Changes:

• Add BaseModel type check
• Use model_dump() for serialization
• Maintain existing behavior for other types

[lfnovo]

Thanks for your contribution @PondererOfEnigmas .. merged.