fix(telegram): 首席架構師 R1 修正 — I-1/I-2/M-1/M-2

I-1: webhooks/sentry_webhook/signoz_webhook 三個呼叫者補 TODO 說明
     無 incident_id 是已知限制（Approval 路徑未建 Incident 關聯）
I-2: TestPushRequest 新增 incident_id 欄位，使 QA 可驗證按鈕渲染
M-1: 移除 _build_inline_keyboard 呼叫中多餘的 `or message.incident_id`
M-2: 補充 900/1000 截斷長度差異說明

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

apps/api/src/services/telegram_gateway.py

@@ -293,7 +293,7 @@ class TelegramMessage:
             f"🔍 <a href='{signoz_url}'>SignOz Trace</a>"
         )
 
-        return message[:900]
+        return message[:900]  # 900 字元上限 (nemotron 版用 1000，因有額外 tool 區塊)
 
     def format_with_nemotron(self) -> str:
         """
@@ -1364,7 +1364,7 @@ class TelegramGateway:
             approval_id=approval_id,
             include_auto_tuning=bool(auto_tuning_command),
             auto_tuning_command=auto_tuning_command,
-            incident_id=incident_id or message.incident_id,
+            incident_id=incident_id,
         )
 
         # 發送訊息