Refactor exception handling in middleware

Updated `HandleExceptionAsync` to set response status
directly for each exception type and streamlined JSON
serialization of error messages.

EnvelopeGenerator.GeneratorAPI/Middleware/ExceptionHandlingMiddleware.cs

@@ -54,37 +54,31 @@ public class ExceptionHandlingMiddleware
     private static async Task HandleExceptionAsync(HttpContext context, Exception exception, ILogger logger)
     {
         context.Response.ContentType = "application/json";
-        var response = context.Response;
 
         string message;
-        int statusCode;
 
         switch (exception)
         {
             case BadRequestException badRequestEx:
-                statusCode = (int)HttpStatusCode.BadRequest;
+                context.Response.StatusCode = (int)HttpStatusCode.BadRequest;
                 message = badRequestEx.Message;
                 break;
 
             case NotFoundException notFoundEx:
-                statusCode = (int)HttpStatusCode.NotFound;
+                context.Response.StatusCode = (int)HttpStatusCode.NotFound;
                 message = notFoundEx.Message;
                 break;
 
             default:
                 logger.LogError(exception, "Unhandled exception occurred.");
-                statusCode = (int)HttpStatusCode.InternalServerError;
+                context.Response.StatusCode = (int)HttpStatusCode.InternalServerError;
                 message = "An unexpected error occurred.";
                 break;
         }
 
-        response.StatusCode = statusCode;
+        await context.Response.WriteAsync(JsonSerializer.Serialize(new
-
-        var result = JsonSerializer.Serialize(new
         {
-            error = message
+            message
-        });
+        }));
-
-        await context.Response.WriteAsync(result);
     }
 }