Refactor service/repository layers to validate single organization/party lookup

[coderabbitai]

Context

During the review of PR #639, it was identified that the controller layer currently throws an InvalidOperationException when multiple parties are found for a single organization number (see DashboardUserContactInformationController).

This validation logic should be moved from the controller to the service/repository layers to ensure proper separation of concerns and consistent error handling across the application.

Expected Behavior

The service/repository layers should enforce that only a single party is returned for a given organization number, handling multiple-party scenarios gracefully at that layer rather than in the controller.

References

• PR: Add endpoint to search for "Your contactinformation for org" #639
• Discussion: Add endpoint to search for "Your contactinformation for org" #639 (comment)
• Requested by: @martivj