chore: fix G118 in chat tests with t.Cleanup instead of nolint

Use t.Cleanup(cancel) so the context is always cancelled at test end,
replacing the nolint suppression. The extraction.go case genuinely
cannot be restructured (cancel stored in struct, called on close).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: cpcloud

internal/app/chat_test.go

@@ -120,9 +120,9 @@ func TestCancellationDuringSQLGeneration(t *testing.T) {
 	m := newTestModel(t)
 	m.openChat()
 
-	_, cancel := context.WithCancel(
-		context.Background(),
-	) //nolint:gosec // cancel stored in CancelFn, called by ctrl+c
+	ctx, cancel := context.WithCancel(context.Background())
+	t.Cleanup(cancel)
+	_ = ctx // only the cancel func is needed; ctx is cleaned up via t.Cleanup
 	m.chat.CurrentQuery = testQuestion
 	m.chat.Streaming = true
 	m.chat.StreamingSQL = true
@@ -152,9 +152,9 @@ func TestCancellationDuringAnswerStreaming(t *testing.T) {
 	m := newTestModel(t)
 	m.openChat()
 
-	_, cancel := context.WithCancel(
-		context.Background(),
-	) //nolint:gosec // cancel stored in CancelFn, called by ctrl+c
+	ctx, cancel := context.WithCancel(context.Background())
+	t.Cleanup(cancel)
+	_ = ctx
 	m.chat.CurrentQuery = testQuestion
 	m.chat.Streaming = true
 	m.chat.StreamingSQL = false // stage 2