Refactor: Rename DocumentOperator to DocumentService

Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.

DocumentService.API/Middleware/ExceptionHandlingMiddleware.cs

@@ -0,0 +1,109 @@
+using DocumentService.Domain.Common.Exceptions;
+using FluentValidation;
+using Microsoft.AspNetCore.Mvc;
+using System.Net;
+using System.Text.Json;
+
+namespace DocumentService.API.Middleware;
+
+/// <summary>
+/// Central exception handling middleware
+/// Maps exceptions to HTTP status codes and RFC 7807 Problem Details
+/// </summary>
+/// <remarks>
+/// Initializes a new instance of the <see cref="ExceptionHandlingMiddleware"/> class.
+/// </remarks>
+/// <param name="Next">The next middleware in the pipeline.</param>
+public class ExceptionHandlingMiddleware(RequestDelegate Next)
+{
+    private static readonly JsonSerializerOptions ProbDetailsJsonOpt = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase
+    };
+
+    /// <summary>
+    /// Invokes the middleware to handle incoming HTTP requests and catch exceptions.
+    /// </summary>
+    /// <param name="context">The HTTP context for the current request.</param>
+    public async Task InvokeAsync(HttpContext context)
+    {
+        try
+        {
+            await Next(context);
+        }
+        catch (Exception ex)
+        {
+            await HandleExceptionAsync(context, ex);
+        }
+    }
+
+    private static async Task HandleExceptionAsync(HttpContext context, Exception exception)
+    {
+        var (statusCode, problemDetails) = MapExceptionToProblemDetails(exception, context);
+
+        context.Response.StatusCode = (int)statusCode;
+        context.Response.ContentType = "application/problem+json";
+
+        await context.Response.WriteAsync(JsonSerializer.Serialize(problemDetails, ProbDetailsJsonOpt));
+    }
+
+    private static (HttpStatusCode StatusCode, ProblemDetails ProblemDetails) MapExceptionToProblemDetails(
+        Exception exception,
+        HttpContext context)
+    {
+        return exception switch
+        {
+            // FluentValidation (400 Bad Request)
+            ValidationException validationEx => (
+                HttpStatusCode.BadRequest,
+                new ProblemDetails
+                {
+                    Type = "https://datatracker.ietf.org/doc/html/rfc7231#section-6.5.1",
+                    Title = "Validation Error",
+                    Status = (int)HttpStatusCode.BadRequest,
+                    Detail = string.Join("; ", validationEx.Errors.Select(e => e.ErrorMessage)),
+                    Instance = context.Request.Path
+                }
+            ),
+
+            // Bad Request Exception (400 Bad Request)
+            BadRequestException badReqEx => (
+                HttpStatusCode.BadRequest,
+                new ProblemDetails
+                {
+                    Type = "https://datatracker.ietf.org/doc/html/rfc7231#section-6.5.4",
+                    Title = "Bad Request",
+                    Status = (int)HttpStatusCode.BadRequest,
+                    Detail = badReqEx.Message,
+                    Instance = context.Request.Path
+                }
+            ),
+
+            // Not Found Exception (404 Not Found)
+            NotFoundException notFoundEx => (
+                HttpStatusCode.NotFound,
+                new ProblemDetails
+                {
+                    Type = "https://datatracker.ietf.org/doc/html/rfc7231#section-6.5.4",
+                    Title = "Resource Not Found",
+                    Status = (int)HttpStatusCode.NotFound,
+                    Detail = notFoundEx.Message,
+                    Instance = context.Request.Path
+                }
+            ),
+
+            // Generic Exception (500 Internal Server Error)
+            _ => (
+                HttpStatusCode.InternalServerError,
+                new ProblemDetails
+                {
+                    Type = "https://datatracker.ietf.org/doc/html/rfc7231#section-6.6.1",
+                    Title = "Internal Server Error",
+                    Status = (int)HttpStatusCode.InternalServerError,
+                    Detail = "An unexpected error occurred. Please contact support.",
+                    Instance = context.Request.Path
+                }
+            )
+        };
+    }
+}