(B) Add verification for unique username on register

Clinics.Backend/Application/Users/Commands/RegisterDoctor/RegisterDoctorHandler.cs

 using Application.Abstractions.CQRS.Commands;
 using Domain.Entities.Identity.Users;
+using Domain.Errors;
 using Domain.Repositories;
 using Domain.Shared;
 using Domain.UnitOfWork;
@@ -30,7 +31,15 @@ public class RegisterDoctorHandler : CommandHandlerBase<RegisterDoctorCommand, R
             return Result.Failure<RegisterDoctorResponse>(doctorUserResult.Error);
         #endregion
 
-        #region 2. Register (save to DB)
+        #region 2. Verify unique username
+        var uniqueUserNameResult = await _userRepository.IsUserNameAvailableAsunc(request.UserName);
+        if (uniqueUserNameResult.IsFailure)
+            return Result.Failure<RegisterDoctorResponse>(uniqueUserNameResult.Error);
+        if (uniqueUserNameResult.Value == false)
+            return Result.Failure<RegisterDoctorResponse>(IdentityErrors.TakenUserName);
+        #endregion
+
+        #region 3. Register (save to DB)
         var registerResult = await _userRepository.RegisterDoctorAsync(doctorUserResult.Value);
         if (registerResult.IsFailure)
             return Result.Failure<RegisterDoctorResponse>(registerResult.Error);

Clinics.Backend/Application/Users/Commands/RegisterReceptionist/RegisterReceptionistCommandHandler.cs

 using Application.Abstractions.CQRS.Commands;
 using Application.Users.Commands.RegisterDoctor;
 using Domain.Entities.Identity.Users;
+using Domain.Errors;
 using Domain.Repositories;
 using Domain.Shared;
 using Domain.UnitOfWork;
@@ -31,7 +32,15 @@ public class RegisterReceptionistCommandHandler : CommandHandlerBase<RegisterRec
             return Result.Failure<RegisterReceptionistResponse>(receptionistUserResult.Error);
         #endregion
 
-        #region 2. Register (save to DB)
+        #region 2. Verify unique username
+        var uniqueUserNameResult = await _userRepository.IsUserNameAvailableAsunc(request.UserName);
+        if (uniqueUserNameResult.IsFailure)
+            return Result.Failure<RegisterReceptionistResponse>(uniqueUserNameResult.Error);
+        if (uniqueUserNameResult.Value == false)
+            return Result.Failure<RegisterReceptionistResponse>(IdentityErrors.TakenUserName);
+        #endregion
+
+        #region 3. Register (save to DB)
         var registerResult = await _userRepository.RegisterReceptionistAsync(receptionistUserResult.Value);
         if (registerResult.IsFailure)
             return Result.Failure<RegisterReceptionistResponse>(registerResult.Error);

Clinics.Backend/Domain/Errors/IdentityErrors.cs

@@ -7,5 +7,6 @@ public static class IdentityErrors
     public static Error InvalidRole => new("Identity.InvalidRole", "Role specified for user is invalid");
     public static Error NotFound => new("Identity.NotFound", "المستخدم غير مسجّل في النظام");
     public static Error PasswordMismatch => new("Identity.PasswordMismatch", "كلمة السر غير صحيحة");
+    public static Error TakenUserName => new("Identity.TakenUserName", "يوجد مستخدم آخر له نفس اسم المستخدم");
     public static Error UnableToRegister => new("Identity.UnableToRegister", "تعذر تسجيل المستخدم");
 }

Clinics.Backend/Domain/Errors/PersistenceErrors.cs

@@ -5,7 +5,7 @@ namespace Domain.Errors;
 public static class PersistenceErrors
 {
     public static Error UnableToCompleteTransaction =>
-        new("Persistence.UnableToCompleteTransaction", "حدثت مشكلة عند الاتصال مع قاعدة البيانات");
+        new("Persistence.UnableToCompleteTransaction", "لم يتم تنفيذ العملية، حدثت مشكلة عند الاتصال مع قاعدة البيانات");
 
     public static Error UnableToCreate =>
         new("Persistence.UnableToCreate", "فشلت عملية الإضافة");
@@ -20,5 +20,5 @@ public static class PersistenceErrors
         new("Persistence.NotFound", "الغرض المطلوب غير موجود");
 
     public static Error Unknown =>
-        new("Persistence.Unknown", "حدث خطأ غير متوقع");
+        new("Persistence.Unknown", "حدثت مشكلة عند الاتصال مع قاعدة البيانات");
 }

Clinics.Backend/Domain/Repositories/IUserRepository.cs

@@ -13,6 +13,10 @@ public interface IUserRepository : IRepository<User>
     public Task<Result<User?>> VerifyPasswordAsync(string userName, string password);
     #endregion
 
+    #region Available username
+    public Task<Result<bool>> IsUserNameAvailableAsunc(string userName);
+    #endregion
+
     #region Doctor users
 
     #region Get doctor user by user name full

Clinics.Backend/Persistence/Repositories/Users/UserRepository.cs

@@ -61,6 +61,21 @@ public class UserRepository : Repositroy<User>, IUserRepository
     }
     #endregion
 
+    #region Available username
+    public async Task<Result<bool>> IsUserNameAvailableAsunc(string userName)
+    {
+        try
+        {
+            var query = _context.Set<User>().Where(user => user.UserName == userName);
+            return (await query.ToListAsync()).Count == 0;
+        }
+        catch (Exception)
+        {
+            return Result.Failure<bool>(PersistenceErrors.Unknown);
+        }
+    }
+    #endregion
+
     #region Doctor users
 
     #region Get doctor user by user name full