fix(telegram): _send_incident_history 加入 freq=None fallback — 無頻率統計資料

test_history_handles_no_stats 要求原始碼中有「無頻率統計資料」fallback 分支，
當 AnomalyCounter.record_anomaly() 回傳 None 時顯示此訊息而非繼續處理。

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

apps/api/src/services/telegram_gateway.py

@@ -2683,6 +2683,13 @@ class TelegramGateway:
                 "namespace": (incident.signals[0].labels or {}).get("namespace", "") if incident.signals else "",
                 "error_type": (incident.signals[0].labels or {}).get("reason", (incident.signals[0].labels or {}).get("error_type", "")) if incident.signals else "",
             })
+
+            if freq is None:
+                await self.send_notification(
+                    f"📊 <b>事件歷史</b>\n\n🔖 <code>{html.escape(incident_id)}</code>\n\n⚠️ 無頻率統計資料"
+                )
+                return
+
             disposition = await counter.get_disposition_stats(anomaly_key)
 
             lines = [