fix(agent-office): dead-letter _dl_notified 갱신을 발송성공 시로 한정 + collect_status 예외방어 (B4 리뷰)

- _dl_notified[name] = dl을 if ok: 블록 안으로 이동 — 텔레그램 실패 시 갱신 방지
- check_and_alert에 collect_status try/except 추가 — 스케줄러 잡 생존 보장
- tests: import app.node_monitor as nm 최상단 이동
- tests: test_dl_notified_not_updated_on_telegram_failure 회귀 테스트 추가

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019LV86jBozkNhSFXJA412fq

agent-office/app/node_monitor.py

@@ -107,7 +107,11 @@ async def check_and_alert(status=None) -> list[str]:
     """
     from .telegram.messaging import send_raw
     from .db import add_log
-    st = status or await collect_status()
+    try:
+        st = status or await collect_status()
+    except Exception:
+        logger.exception("collect_status 예외")
+        return []
     sent: list[str] = []
     for w in st["workers"]:
         name, alive = w["name"], w.get("alive", False)
@@ -129,7 +133,7 @@ async def check_and_alert(status=None) -> list[str]:
             if (await send_raw(text=text)).get("ok"):
                 add_log("node_monitor", f"{name} dead-letter {dl}", "warning")
                 sent.append(text)
-            _dl_notified[name] = dl
+                _dl_notified[name] = dl
         elif dl == 0:
             _dl_notified.pop(name, None)
     return sent