T4: nothing answers 500

NeverFiveHundredTests walks every endpoint the server maps and fills each route parameter
with junk ('x', zeroed and random ObjectIds, a dot-dot segment, 5000 characters). It calls
each one anonymously, as a persona, and with a junk token, and asserts that nothing
answers 5xx or throws, and that every /api error carries a Mastodon error body.

It found two bugs:
- A junk 'Authorization: Bearer' on any non-/api route, anonymous ones included
  (/build.json, /peasants/*, inboxes), broke the response. JwtEvents.AuthenticationFailed
  wrote a body during authentication and the endpoint then wrote its own. The 401 body now
  comes from the Challenge event, which runs only when an endpoint needs a user.
- /api answered 401, 404 and 429 with no body. UseMastodonErrorBodies gives any /api error
  that leaves without a body Mastodon's {"error": ...}.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ELjqpznMFMNrJoJUj6K5p2

PrivaPub/Api/Mastodon/Infrastructure/MastodonErrors.cs

@@ -0,0 +1,29 @@
+using System.Text.Json;
+
+namespace PrivaPub.Api.Mastodon.Infrastructure
+{
+	public static class MastodonErrors
+	{
+		public static IApplicationBuilder UseMastodonErrorBodies(this IApplicationBuilder app) =>
+			app.UseWhen(context => context.Request.Path.StartsWithSegments("/api"), api => api.UseStatusCodePages(async pages =>
+			{
+				var response = pages.HttpContext.Response;
+				response.ContentType = "application/json; charset=utf-8";
+				await response.WriteAsync(JsonSerializer.Serialize(new { error = Message(response.StatusCode) }, MastodonJson.Options));
+			}));
+
+		public static string Message(int status) => status switch
+		{
+			StatusCodes.Status401Unauthorized => "The access token is invalid",
+			StatusCodes.Status403Forbidden => "This action is outside the authorized scopes",
+			StatusCodes.Status404NotFound => "Record not found",
+			StatusCodes.Status405MethodNotAllowed => "Method not allowed",
+			StatusCodes.Status413PayloadTooLarge => "Payload too large",
+			StatusCodes.Status415UnsupportedMediaType => "Unsupported media type",
+			StatusCodes.Status422UnprocessableEntity => "Validation failed",
+			StatusCodes.Status429TooManyRequests => "Too many requests",
+			>= 500 => "Something went wrong.",
+			_ => "Bad request"
+		};
+	}
+}