Answer 499 to a client that goes away while waiting for an image (closes #339)

When a request waits for an image another request is making and its
client goes away, the image proxy returns context.Canceled. serveImage
now answers it 499, the status nginx calls "client closed request". It
logs it once at info level with the request_id, host and path. It was
answered 500 and logged at error level, as pixa's own failure. A
cancellation while the request's context is live is still answered 500.
The request making the image is still answered as before. README.md
lists the 499 on both image routes.

Model: opus-5-5

internal/handlers/client_gone_internal_test.go

@@ -58,13 +58,14 @@ func readLogLines(t *testing.T, logs *bytes.Buffer) []map[string]any {
 	return lines
 }
 
-// TestImageRoutes_ClientGoneAnswers499 checks that both image routes answer
+// TestImageRoutes_ClientGoneWhileWaitingAnswers499 checks that both image
-// 499 to a request whose client goes away while its image is being made. A
+// routes answer 499 to a request whose client goes away while it waits for an
-// first request's fetch is held, so the image is being made, and a second
+// image another request is making. A first request's fetch is held, so the
-// request for the same image waits for it until its client goes away. The
+// image is being made, and a second request for the same image waits for it
-// second request must be logged once at info level with its request ID and
+// until its client goes away. The second request must be logged once at info
-// the source host and path. Nothing may be logged at error level.
+// level with its request ID and the source host and path. Nothing may be
-func TestImageRoutes_ClientGoneAnswers499(t *testing.T) {
+// logged at error level.
+func TestImageRoutes_ClientGoneWhileWaitingAnswers499(t *testing.T) {
 	t.Parallel()
 
 	const requestID = "test-request-id"