fix(app): Error details missing when user type is unknown (#1493)

<!--- Provide a general summary of your changes in the Title above -->

## Description

<!--- Describe your changes in detail -->
Errors list is missing in error details when user type is unknown

## Related Issue(s)

- #1491 

## Verification

- [x] **Your** code builds clean without any errors or warnings
- [x] Manual testing done (required)
- [ ] Relevant automated test added (if you find this hard, leave it and
we'll help out)

## Documentation

- [ ] Documentation is updated (either in `docs`-directory, Altinnpedia
or a separate linked PR in
[altinn-studio-docs.](https://github.com/Altinn/altinn-studio-docs), if
applicable)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Bug Fixes**
- Improved handling of unknown user types in the authentication
middleware, ensuring clearer response management.
  
- **Refactor**
- Enhanced code clarity by separating response retrieval and writing
processes in the user type validation logic.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/Digdir.Domain.Dialogporten.WebApi/Common/Authentication/UserTypeValidationMiddleware.cs

@@ -21,13 +21,14 @@ public async Task InvokeAsync(HttpContext context)
             if (userType == UserIdType.Unknown)
             {
                 context.Response.StatusCode = StatusCodes.Status403Forbidden;
-                await context.Response.WriteAsJsonAsync(context.GetResponseOrDefault(
+                var response = context.GetResponseOrDefault(
                     context.Response.StatusCode,
                     [
                         new("Type",
                             "The request was authenticated, but we were unable to determine valid user type (person or system user) in order to authorize the request.")
                     ]
-                ));
+                );
+                await context.Response.WriteAsJsonAsync(response, response.GetType());
 
                 return;
             }