Improve exception handling and validation responses

Updated `ExceptionHandlingMiddleware` to enhance error handling:
- Added `Microsoft.AspNetCore.Mvc` and `System.Text.Json` for structured JSON responses.
- Made `message` nullable to handle cases without explicit messages.
- Refactored `ValidationException` handling:
  - Changed HTTP status code from 422 to 400.
  - Grouped validation errors by property and returned them in a `ValidationProblemDetails` object.
- Conditionally wrote `message` to the response only if not null.
- Improved structure and clarity of validation error responses.

src/ReC.API/Middleware/ExceptionHandlingMiddleware.cs

@@ -1,5 +1,6 @@
 using DigitalData.Core.Exceptions;
 using FluentValidation;
+using Microsoft.AspNetCore.Mvc;
 using ReC.Application.Common.Exceptions;
 using System.Net;
 using System.Text.Json;
@@ -57,7 +58,7 @@ public class ExceptionHandlingMiddleware
     {
         context.Response.ContentType = "application/json";
 
-        string message;
+        string? message = null;
 
         switch (exception)
         {
@@ -67,8 +68,20 @@ public class ExceptionHandlingMiddleware
                 break;
 
             case ValidationException validationEx:
-                context.Response.StatusCode = (int)HttpStatusCode.UnprocessableEntity;
-                message = "Validation failed: " + string.Join("; ", validationEx.Errors.Select(e => e.ErrorMessage));
+                context.Response.StatusCode = (int)HttpStatusCode.BadRequest;
+
+                var errors = validationEx.Errors
+                    .GroupBy(e => e.PropertyName, e => e.ErrorMessage)
+                    .ToDictionary(failureGroup => failureGroup.Key, failureGroup => failureGroup.ToArray());
+
+                await context.Response.WriteAsJsonAsync(new ValidationProblemDetails()
+                {
+                    Title = "Validation failed",
+                    Errors = validationEx.Errors
+                        .GroupBy(e => e.PropertyName, e => e.ErrorMessage)
+                        .ToDictionary(failureGroup => failureGroup.Key, failureGroup => failureGroup.ToArray()),
+                });
+
                 break;
 
             case NotFoundException notFoundEx:
@@ -88,6 +101,7 @@ public class ExceptionHandlingMiddleware
                 break;
         }
 
+        if (message is not null)
             await context.Response.WriteAsync(JsonSerializer.Serialize(new
             {
                 message