test(tui): drop racy tick-cmd assertion from slack subscription test

TestBottomSlackAnimationSubscribesWhileDecaying was flaky when run
in parallel with other tests touching the animation coordinator.

animation.Subscription.Start only returns a non-nil tea.Cmd for the
first global subscriber: subsequent subscribers piggy-back on the
already-running tick stream. So when other parallel tests had
already registered an animation, our Start returned nil even though
our subscription was correctly registered, and the
assert.NotNil(t, cmd, ...) addition from the previous commit failed
roughly 1 in 5 runs.

The local IsActive() assertion is sufficient to verify that
handleAnimationTick registered the subscription. Asserting on the
global tick command is racy without exposing test-only resets in
the animation package, which isn't worth it.

Verified stable across 20 -race iterations.

Assisted-By: docker-agent

Author: dgageot

pkg/tui/components/messages/slack_test.go

@@ -111,14 +111,14 @@ func TestBottomSlackAnimationSubscribesWhileDecaying(t *testing.T) {
 		"no slack means no animation subscription")
 
 	// Slack > 0: a tick should subscribe so further ticks keep firing
-	// even after fade animations finish, and Update must return the
-	// scheduling command so Bubbletea actually delivers the next tick.
+	// even after fade animations finish. We can only assert on the local
+	// subscription state — the global tick command from Subscription.Start
+	// is non-nil only for the first global subscriber, which is racy when
+	// tests touching the animation coordinator run in parallel.
 	m.bottomSlack = 2
-	_, cmd := m.Update(animation.TickMsg{Frame: 1})
+	m.Update(animation.TickMsg{Frame: 1})
 	assert.True(t, m.slackAnimationSub.IsActive(),
 		"subscription should be active while slack > 0")
-	assert.NotNil(t, cmd,
-		"Update must return a tick command so the next tick is scheduled")
 
 	// Once slack hits zero, the subscription must release the global tick.
 	m.Update(animation.TickMsg{Frame: 2})