refactor: improve exception middleware safety and RFC 7807 compliance

- Add HasStarted check before modifying response
- Prevent errors when exception occurs after headers sent
- Log warning when unable to write error response
- Use authoritative Mozilla documentation for error types

Author: nanotaboada

src/Dotnet.Samples.AspNetCore.WebApi/Extensions/MiddlewareExtensions.cs

@@ -11,8 +11,8 @@ public static class MiddlewareExtensions
     /// Adds global exception handling middleware to the application pipeline.
     /// This middleware catches unhandled exceptions and returns RFC 7807 compliant error responses.
     /// </summary>
-    /// <param name="app">The web application builder.</param>
-    /// <returns>The web application builder for method chaining.</returns>
+    /// <param name="app">The web application used to configure the HTTP pipeline, and routes.</param>
+    /// <returns>The WebApplication object for method chaining.</returns>
     public static WebApplication UseExceptionHandling(this WebApplication app)
     {
         app.UseMiddleware<ExceptionMiddleware>();

src/Dotnet.Samples.AspNetCore.WebApi/Middlewares/ExceptionMiddleware.cs

@@ -35,13 +35,13 @@ public async Task InvokeAsync(HttpContext context, RequestDelegate next)
     /// </summary>
     private async Task HandleExceptionAsync(HttpContext context, Exception exception)
     {
-        var (statusCode, title) = MapExceptionToStatusCode(exception);
+        var (status, title) = MapExceptionToStatusCode(exception);
 
         var problemDetails = new ProblemDetails
         {
-            Type = $"https://httpstatuses.com/{statusCode}",
+            Type = $"https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/{status}",
             Title = title,
-            Status = statusCode,
+            Status = status,
             Detail = GetExceptionDetail(exception),
             Instance = context.Request.Path
         };
@@ -55,13 +55,26 @@ private async Task HandleExceptionAsync(HttpContext context, Exception exception
             "Unhandled exception occurred. TraceId: {TraceId}, Path: {Path}, StatusCode: {StatusCode}",
             context.TraceIdentifier,
             context.Request.Path,
-            statusCode
+            status
         );
 
-        context.Response.StatusCode = statusCode;
-        context.Response.ContentType = ProblemDetailsContentType;
+        // Only modify response if headers haven't been sent yet
+        if (!context.Response.HasStarted)
+        {
+            context.Response.StatusCode = status;
+            context.Response.ContentType = ProblemDetailsContentType;
 
-        await context.Response.WriteAsync(JsonSerializer.Serialize(problemDetails, JsonOptions));
+            await context.Response.WriteAsync(
+                JsonSerializer.Serialize(problemDetails, JsonOptions)
+            );
+        }
+        else
+        {
+            logger.LogWarning(
+                "Unable to write error response for TraceId: {TraceId}. Response has already started.",
+                context.TraceIdentifier
+            );
+        }
     }
 
     /// <summary>