fix(ai): Ask 跟进换表禁止默认 created_at

多轮/换表后列未注入时只澄清或 DESCRIBE，不得编时间列写业务 SQL。

server/internal/ai/ai_test.go

@@ -124,6 +124,9 @@ func TestAskChatsWithoutClaimingDatabaseExecution(t *testing.T) {
 	if !strings.Contains(generate[0].Content, "禁止编造") || !strings.Contains(generate[0].Content, "UPDATE_TIME") {
 		t.Fatalf("system rules missing invent/UPDATE_TIME guards: %s", generate[0].Content)
 	}
+	if !strings.Contains(generate[0].Content, "换表") || !strings.Contains(generate[0].Content, "created_at") {
+		t.Fatalf("follow-up no-invent column rules missing: %s", generate[0].Content)
+	}
 	offline := chatStub(request)
 	if offline.SQL != "" || !strings.Contains(offline.Content, "离线体验无法回答") {
 		t.Fatalf("offline Ask must not pretend to generate or query: %+v", offline)