Add ValidatePDF feature with API, Swagger, and tests

Implemented the ValidatePDF feature end-to-end:
- Added `/api/v1/documents/validate` Minimal API endpoint.
- Introduced centralized ExceptionHandlingMiddleware.
- Configured Swagger with `AddSwaggerDocumentation` extension.
- Enabled XML comments in `DocumentOperator.API.csproj`.
- Updated DTOs with XML comments and added `FileSizeMB`.
- Added integration tests for the ValidatePDF endpoint (3 tests).
- Registered infrastructure services (e.g., `IPdfProcessor`).
- Refactored `Program.cs` to include middleware and endpoints.
- Updated PHASENPLAN.md and ROADMAP.md to reflect progress.
- Cleaned up code and made `Program` accessible for tests.

DocumentOperator.API/Middleware/ExceptionHandlingMiddleware.cs

@@ -1,6 +1,124 @@
-namespace DocumentOperator.API.Middleware
+using DocumentOperator.Domain.Common.Exceptions;
+using FluentValidation;
+using Microsoft.AspNetCore.Mvc;
+using System.Net;
+using System.Text.Json;
+
+namespace DocumentOperator.API.Middleware;
+
+/// <summary>
+/// Central exception handling middleware
+/// Maps exceptions to HTTP status codes and RFC 7807 Problem Details
+/// </summary>
+public class ExceptionHandlingMiddleware
 {
-    public class ExceptionHandlingMiddleware
+    private readonly RequestDelegate _next;
+    private readonly ILogger<ExceptionHandlingMiddleware> _logger;
+
+    public ExceptionHandlingMiddleware(RequestDelegate next, ILogger<ExceptionHandlingMiddleware> logger)
     {
+        _next = next;
+        _logger = logger;
+    }
+
+    public async Task InvokeAsync(HttpContext context)
+    {
+        try
+        {
+            await _next(context);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Unhandled exception: {ExceptionMessage}", ex.Message);
+            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";
+
+        var options = new JsonSerializerOptions
+        {
+            PropertyNamingPolicy = JsonNamingPolicy.CamelCase
+        };
+
+        await context.Response.WriteAsync(JsonSerializer.Serialize(problemDetails, options));
+    }
+
+    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
+                }
+            ),
+
+            // Domain Validation Exception (400 Bad Request)
+            DomainValidationException domainEx => (
+                HttpStatusCode.BadRequest,
+                new ProblemDetails
+                {
+                    Type = "https://datatracker.ietf.org/doc/html/rfc7231#section-6.5.1",
+                    Title = "Domain Validation Error",
+                    Status = (int)HttpStatusCode.BadRequest,
+                    Detail = domainEx.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
+                }
+            ),
+
+            // PDF Processing Exception (500 Internal Server Error)
+            PdfProcessingException pdfEx => (
+                HttpStatusCode.InternalServerError,
+                new ProblemDetails
+                {
+                    Type = "https://datatracker.ietf.org/doc/html/rfc7231#section-6.6.1",
+                    Title = "PDF Processing Error",
+                    Status = (int)HttpStatusCode.InternalServerError,
+                    Detail = pdfEx.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
+                }
+            )
+        };
     }
 }