Fix JSON serialization issue with date fields

[agamm]

Summary

• Fixes JSON serialization failure when Pydantic models contain date fields
• Updates JobResult.to_dict() to use model_dump(mode='json') for proper serialization
• Adds comprehensive unit tests for date field serialization scenarios

Problem

When using Pydantic models with date fields as response_model, the library would fail to save results to JSON files because Python's json.dump() cannot serialize date objects directly. This caused truncated output files and error messages like "Object of type date is not JSON serializable".

Solution

Changed model_dump() to model_dump(mode='json') in JobResult.to_dict(), which ensures all fields are JSON-serializable (dates become ISO format strings).

Test Plan

• Added unit tests for date field serialization
• Verified fix works with both Pydantic models and dict responses
• All existing tests continue to pass

No breaking changes