test(similarity): update default-batch assertion 5→25

Follows ca1bc5a, which raised the worker batch default. The defaults
test pinned the old value; align it with the intended new default.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/similarity/worker_test.go

@@ -13,8 +13,8 @@ func TestNewWorker_DefaultsMatchSpec(t *testing.T) {
 	if w.tick != 1*time.Hour {
 		t.Errorf("tick = %v, want 1h", w.tick)
 	}
-	if w.batch != 5 {
-		t.Errorf("batch = %d, want 5", w.batch)
+	if w.batch != 25 {
+		t.Errorf("batch = %d, want 25", w.batch)
 	}
 	if w.topK != 20 {
 		t.Errorf("topK = %d, want 20", w.topK)