🐛 fix: resolve ValidationException ambiguity and null reference issues

- Use fully qualified FluentValidation.ValidationException in command handlers
  to resolve ambiguity with Luminous.Application.Common.Exceptions.ValidationException
- Make LinkedDeviceDto.RefreshToken nullable to match AuthResultDto.RefreshToken

src/Luminous.Application/DTOs/DeviceDto.cs

@@ -62,7 +62,7 @@ public sealed record LinkedDeviceDto
 {
     public DeviceDto Device { get; init; } = new();
     public string AccessToken { get; init; } = string.Empty;
-    public string RefreshToken { get; init; } = string.Empty;
+    public string? RefreshToken { get; init; }
     public string TokenType { get; init; } = "Bearer";
     public int ExpiresIn { get; init; }
 }

src/Luminous.Application/Features/Devices/Commands/LinkDeviceCommand.cs

@@ -65,7 +65,7 @@ public async Task<LinkedDeviceDto> Handle(LinkDeviceCommand request, Cancellatio
         // Verify link code is still valid
         if (!device.IsLinkCodeValid)
         {
-            throw new ValidationException([new FluentValidation.Results.ValidationFailure(
+            throw new FluentValidation.ValidationException([new FluentValidation.Results.ValidationFailure(
                 "LinkCode", "Link code has expired. Please generate a new code.")]);
         }
 

src/Luminous.Application/Features/Devices/Commands/RecordHeartbeatCommand.cs

@@ -56,7 +56,7 @@ public async Task<DeviceHeartbeatDto> Handle(RecordHeartbeatCommand request, Can
 
         if (!device.IsLinked)
         {
-            throw new ValidationException([new FluentValidation.Results.ValidationFailure(
+            throw new FluentValidation.ValidationException([new FluentValidation.Results.ValidationFailure(
                 "DeviceId", "Device is not linked to any family.")]);
         }
 

src/Luminous.Application/Features/Devices/Commands/UnlinkDeviceCommand.cs

@@ -51,7 +51,7 @@ public async Task<Unit> Handle(UnlinkDeviceCommand request, CancellationToken ca
 
         if (!device.IsLinked)
         {
-            throw new ValidationException([new FluentValidation.Results.ValidationFailure(
+            throw new FluentValidation.ValidationException([new FluentValidation.Results.ValidationFailure(
                 "DeviceId", "Device is not linked to any family.")]);
         }