Say govips gives pixa no way to stop an encode; give AVIF's worst case

libvips can stop a running encode, but govips gives pixa no way to ask
it to, which README.md "Routes" and the Process and encode comments
now say in place of "libvips cannot stop an encode". The avifEffort
comment gives its 51 seconds as the RGB time and 110 seconds with an
alpha channel as the worst case, as README.md does.

Model: opus-5-5

internal/handlers/server_busy_internal_test.go

@@ -49,38 +49,39 @@ func TestServerBusyAnswers503(t *testing.T) {
 	}
 }
 
-// TestProcessingTimedOutAnswers503 checks that both image routes answer 503
-// with the server busy error when the image service reports that processing
-// stopped at the deadline of the request doing it, wrapped as the service
-// wraps it. A request that waited for that processing gets this error while
-// its own deadline may have time left.
+// TestProcessingTimedOutAnswers503 checks that respondImageError and
+// handleImageError, which answer the errors of the two image routes, answer
+// 503 with the server busy error when the image service reports that
+// processing stopped at the deadline of the request doing it, wrapped as the
+// service wraps it. A request that waited for that processing gets this error
+// while its own deadline may have time left. The request's source only goes
+// into a log line, so it is left empty.
 func TestProcessingTimedOutAnswers503(t *testing.T) {
 	t.Parallel()
 
 	h := &Handlers{log: slog.New(slog.DiscardHandler)}
-	req := &imgcache.ImageRequest{SourceHost: "img.example.com", SourcePath: "/a.jpg"}
 	err := fmt.Errorf("%w: failed to encode: %w",
 		imgcache.ErrProcessingTimedOut, context.DeadlineExceeded)
 
 	plain := httptest.NewRecorder()
-	h.respondImageError(plain, req, err)
+	h.respondImageError(plain, &imgcache.ImageRequest{}, err)
 
 	encrypted := httptest.NewRecorder()
 	h.handleImageError(encrypted, err)
 
-	for route, rec := range map[string]*httptest.ResponseRecorder{
-		"/v1/image/": plain, "/v1/e/": encrypted,
+	for name, rec := range map[string]*httptest.ResponseRecorder{
+		"respondImageError": plain, "handleImageError": encrypted,
 	} {
-		t.Logf("%s: status = %d, body = %q", route, rec.Code, rec.Body.String())
+		t.Logf("%s: status = %d, body = %q", name, rec.Code, rec.Body.String())
 
 		if rec.Code != http.StatusServiceUnavailable {
 			t.Errorf("%s: status = %d, want %d",
-				route, rec.Code, http.StatusServiceUnavailable)
+				name, rec.Code, http.StatusServiceUnavailable)
 		}
 
 		if !strings.Contains(rec.Body.String(), "server busy, try again later") {
 			t.Errorf("%s: body = %q, want the server busy error",
-				route, rec.Body.String())
+				name, rec.Body.String())
 		}
 	}
 }