Implement comprehensive validation for osbl_data structure

## Context
Currently, the OsblCreation model has minimal validation for the osbl_data JSON field. We need to implement comprehensive validation to ensure data integrity and prevent invalid data structures.

## Requirements
- Validate all required fields within osbl_data
- Validate data types for each field
- Validate nested structures (e.g., osbls_causes_attributes)
- Add custom error messages for each validation failure
- Consider using JSON Schema for validation

## Related Links
- PR: https://github.com/Benefactorum/app/pull/159
- Comment: https://github.com/Benefactorum/app/pull/159#discussion_r1943253289

## Notes
This validation is crucial for maintaining data consistency and preventing issues with downstream processing.