Add global exception handling middleware

Introduced ExceptionHandlingMiddleware to handle exceptions across the ASP.NET Core request pipeline. The middleware logs exceptions and returns JSON error responses with appropriate HTTP status codes for BadRequestException (400), NotFoundException (404), and generic errors (500). Dependency injection is used for RequestDelegate and ILogger.

EnvelopeGenerator.ServiceHost/Middleware/ExceptionHandlingMiddleware.cs

@@ -0,0 +1,75 @@
+namespace EnvelopeGenerator.ServiceHost.Middleware;
+
+using DigitalData.Core.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>
+/// <remarks>
+/// Initializes a new instance of the <see cref="ExceptionHandlingMiddleware"/> class.
+/// </remarks>
+/// <param name="next">The next middleware in the request pipeline.</param>
+/// <param name="logger">The logger instance for logging exceptions.</param>
+public class ExceptionHandlingMiddleware(RequestDelegate next, ILogger<ExceptionHandlingMiddleware> 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";
+
+        string message;
+
+        switch (exception)
+        {
+            case BadRequestException badRequestEx:
+                context.Response.StatusCode = (int)HttpStatusCode.BadRequest;
+                message = badRequestEx.Message;
+                break;
+
+            case NotFoundException notFoundEx:
+                context.Response.StatusCode = (int)HttpStatusCode.NotFound;
+                message = notFoundEx.Message;
+                break;
+
+            default:
+                logger.LogError(exception, "Unhandled exception occurred.");
+                context.Response.StatusCode = (int)HttpStatusCode.InternalServerError;
+                message = "An unexpected error occurred.";
+                break;
+        }
+
+        await context.Response.WriteAsync(JsonSerializer.Serialize(new
+        {
+            message
+        }));
+    }
+}