fix(tgbot): let a screen-art reset outlast a probe still in flight

TestScreenArtReprobeWindow failed under -race -shuffle in CI: the probe
slot is process-wide, and a probe an earlier test started against its
own (now closing) server still held it when the next test asked for a
re-probe, so that request was dropped and the stale avatar never
re-probed. The leftover probe could also write its result over the
state the next test had just reset.

Probes now run in a WaitGroup that resetScreenArt waits on, so a reset
between mock servers always starts from an idle probe slot.

internal/web/service/tgbot/screens_art.go

@@ -32,6 +32,8 @@ var (
 	artProbedAt   time.Time
 	// artProbeRunning keeps a burst of renders from starting a burst of probes.
 	artProbeRunning atomic.Bool
+	// artProbes lets resetScreenArt outlast a probe still running against a previous bot.
+	artProbes sync.WaitGroup
 
 	blackTileOnce  sync.Once
 	blackTileBytes []byte
@@ -109,10 +111,10 @@ func (t *Tgbot) ensureScreenArt() {
 	if !artProbeRunning.CompareAndSwap(false, true) {
 		return
 	}
-	go func() {
+	artProbes.Go(func() {
 		defer artProbeRunning.Store(false)
 		t.resolveScreenArt()
-	}()
+	})
 }
 
 // artFileID returns the cached file_id for a screen kind. A fallback tile cached
@@ -163,6 +165,7 @@ func cacheArtFileID(kind, fileID string) {
 
 // resetScreenArt drops the cached file_ids; tests use it between mock servers.
 func resetScreenArt() {
+	artProbes.Wait()
 	screenArtMu.Lock()
 	screenArtIDs = map[string]string{}
 	artOverrides = map[string]string{}

internal/web/service/tgbot/screens_test.go

@@ -508,6 +508,34 @@ func TestScreenArtReprobeWindow(t *testing.T) {
 	}
 }
 
+// A probe a previous test left in flight held the process-wide probe slot, so under -race -shuffle
+// the next test's stale avatar was never re-probed (CI flake in TestScreenArtReprobeWindow).
+func TestScreenArtResetWaitsForAProbeLeftInFlight(t *testing.T) {
+	slow := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		time.Sleep(300 * time.Millisecond)
+		http.Error(w, "gone", http.StatusServiceUnavailable)
+	}))
+	defer slow.Close()
+	swapTestBot(t, slow.URL)
+	(&Tgbot{}).ensureScreenArt()
+
+	srv, calls := recordingServer(t, false)
+	defer srv.Close()
+	swapTestBot(t, srv.URL)
+	screenArtMu.Lock()
+	artProbedAt = time.Now().Add(-2 * time.Minute)
+	screenArtMu.Unlock()
+
+	(&Tgbot{}).ensureScreenArt()
+	for range 100 {
+		if countMethod(calls(), "getUserProfilePhotos") > 0 {
+			return
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	t.Fatal("the probe left in flight by the previous bot kept the new one from probing")
+}
+
 func TestScreenKeepsArtFromTheBotAvatar(t *testing.T) {
 	tb, calls := newScreenTgbot(t, false)
 	tb.resolveScreenArt()