Add global exception handling middleware

Introduce `ExceptionHandlingMiddleware` to capture and log exceptions globally in the application. This middleware returns appropriate JSON error responses based on specific exception types, such as `BadRequestException` and `NotFoundException`. The middleware is registered in `Program.cs` to ensure it processes all HTTP requests.

EnvelopeGenerator.GeneratorAPI/Middleware/ExceptionHandlingMiddleware.cs

@@ -0,0 +1,90 @@
+namespace EnvelopeGenerator.GeneratorAPI.Middleware;
+
+using EnvelopeGenerator.Application.Exceptions;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.Logging;
+using System.Net;
+using System.Text.Json;
+
+/// <summary>
+/// Middleware for handling exceptions globally in the application.
+/// Captures exceptions thrown during the request pipeline execution,
+/// logs them, and returns an appropriate HTTP response with a JSON error message.
+/// </summary>
+public class ExceptionHandlingMiddleware
+{
+    private readonly RequestDelegate _next;
+    private readonly ILogger<ExceptionHandlingMiddleware> _logger;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ExceptionHandlingMiddleware"/> class.
+    /// </summary>
+    /// <param name="next">The next middleware in the request pipeline.</param>
+    /// <param name="logger">The logger instance for logging exceptions.</param>
+    public ExceptionHandlingMiddleware(RequestDelegate next, ILogger<ExceptionHandlingMiddleware> logger)
+    {
+        _next = next;
+        _logger = logger;
+    }
+
+    /// <summary>
+    /// Invokes the middleware to handle the HTTP request.
+    /// </summary>
+    /// <param name="context">The HTTP context of the current request.</param>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    public async Task InvokeAsync(HttpContext context)
+    {
+        try
+        {
+            await _next(context); // Continue down the pipeline
+        }
+        catch (Exception ex)
+        {
+            await HandleExceptionAsync(context, ex, _logger);
+        }
+    }
+
+    /// <summary>
+    /// Handles exceptions by logging them and writing an appropriate JSON response.
+    /// </summary>
+    /// <param name="context">The HTTP context of the current request.</param>
+    /// <param name="exception">The exception that occurred.</param>
+    /// <param name="logger">The logger instance for logging the exception.</param>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    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;
+                message = badRequestEx.Message;
+                break;
+
+            case NotFoundException notFoundEx:
+                statusCode = (int)HttpStatusCode.NotFound;
+                message = notFoundEx.Message;
+                break;
+
+            default:
+                logger.LogError(exception, "Unhandled exception occurred.");
+                statusCode = (int)HttpStatusCode.InternalServerError;
+                message = "An unexpected error occurred.";
+                break;
+        }
+
+        response.StatusCode = statusCode;
+
+        var result = JsonSerializer.Serialize(new
+        {
+            error = message
+        });
+
+        await context.Response.WriteAsync(result);
+    }
+}

EnvelopeGenerator.GeneratorAPI/Program.cs

@@ -15,6 +15,7 @@ using EnvelopeGenerator.GeneratorAPI.Models;
 using Microsoft.AspNetCore.Authentication.JwtBearer;
 using Microsoft.IdentityModel.Tokens;
 using DigitalData.Core.Abstractions.Security.Extensions;
+using EnvelopeGenerator.GeneratorAPI.Middleware;
 
 var builder = WebApplication.CreateBuilder(args);
 
@@ -169,6 +170,8 @@ var app = builder.Build();
 
 deferredProvider.Factory = () => app.Services;
 
+app.UseMiddleware<ExceptionHandlingMiddleware>();
+
 app.MapOpenApi();
 
 // Configure the HTTP request pipeline.