feat: prevent duplicate employer registration in workshop

CompanyManagment.Application/EmployerApplication.cs

@@ -1340,6 +1340,10 @@ public class EmployerApplication : IEmployerApplication
         var workshopDetails =
             await _institutionContractRepository.GetInstitutionWorkshopInitialDetails(command
                 .InstitutionWorkshopInitialId);
+        if (workshopDetails.Employers.Any(x => x.EmployerId == employer.id))
+        {
+            throw new BadRequestException("این کارفرما قبلا در این کارگاه ثبت شده است");
+        }
         workshopDetails.AddEmployer(employer.id);
 
         await _institutionContractRepository.SaveChangesAsync();