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/Services/JwtEvents.cs

@@ -14,8 +14,11 @@ namespace PrivaPub.Services
 		ILogger<JwtEvents> _logger { get; set; }
 		const string contentType = "application/json";
 
-		public override async Task AuthenticationFailed(AuthenticationFailedContext context)
+		public override async Task Challenge(JwtBearerChallengeContext context)
 		{
+			context.HandleResponse();
+			context.Response.StatusCode = StatusCodes.Status401Unauthorized;
+			context.Response.Headers.WWWAuthenticate = context.AuthenticateFailure == default ? "Bearer" : "Bearer error=\"invalid_token\"";
 			try
 			{
 				var localizer = context.HttpContext.RequestServices.GetRequiredService<IStringLocalizer<GenericRes>>();
@@ -26,7 +29,7 @@ namespace PrivaPub.Services
 			catch (Exception ex)
 			{
 				_logger = context.HttpContext.RequestServices.GetRequiredService<ILogger<JwtEvents>>();
-				_logger.LogError(ex, "Error at AuthenticationFailed()");
+				_logger.LogError(ex, "Error at Challenge()");
 			}
 		}
 
@@ -42,7 +45,7 @@ namespace PrivaPub.Services
 			catch (Exception ex)
 			{
 				_logger = context.HttpContext.RequestServices.GetRequiredService<ILogger<JwtEvents>>();
-				_logger.LogError(ex, "Error at AuthenticationFailed()");
+				_logger.LogError(ex, "Error at Forbidden()");
 			}
 		}
 	}