fix(agent-office): node_monitor 루프 예외 방어 + 테스트 보강 (B2 리뷰)

- per-worker 루프 전체를 try/except로 감싸 Redis 예외 시 redis_ok=False+break (Blocker)
- heartbeat 파싱 except에 UnicodeDecodeError 추가 (Important)
- hb.get('ts') or '' 로 null ts 안전 처리 (Minor)
- 테스트 3개 추가: paused 폴백·processing 집계·llen 예외 회귀

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

agent-office/tests/test_node_monitor.py

@@ -62,3 +62,37 @@ async def test_trader_http_pull_link():
     st = await node_monitor.collect_status(redis=r)
     link = next(l for l in st["links"] if l["from"] == "ai_trade")
     assert link["type"] == "http-pull" and link["status"] == "healthy"
+
+@pytest.mark.asyncio
+async def test_paused_no_watcher_heartbeat_fallback_reason():
+    """paused=True인데 watcher heartbeat 없으면 paused_reason == 'trading' 폴백."""
+    r = FakeRedis(kv={"queue:paused": b"1"})  # watcher heartbeat 없음
+    st = await node_monitor.collect_status(redis=r)
+    assert st["paused"] is True
+    assert st["paused_reason"] == "trading"
+
+@pytest.mark.asyncio
+async def test_processing_count_image_render():
+    """processing:<queue>:<worker_id> 리스트가 있으면 processing 필드에 합산된다."""
+    worker_id = "abc123"
+    proc_key = f"processing:queue:image-render:{worker_id}"
+    r = FakeRedis(
+        kv={"worker:image-render:heartbeat": _hb("image-render", "render", "busy")},
+        lists={proc_key: 3},
+    )
+    st = await node_monitor.collect_status(redis=r)
+    img = next(w for w in st["workers"] if w["name"] == "image-render")
+    assert img["processing"] == 3
+
+@pytest.mark.asyncio
+async def test_llen_exception_returns_redis_ok_false():
+    """워커 루프 중 llen 예외 발생 시 예외를 전파하지 않고 redis_ok=False 반환 (Blocker 회귀)."""
+    class BrokenLlenRedis(FakeRedis):
+        async def llen(self, key):
+            raise ConnectionError("Redis 연결 끊김")
+
+    r = BrokenLlenRedis(
+        kv={"worker:music-render:heartbeat": _hb("music-render", "render", "idle")}
+    )
+    st = await node_monitor.collect_status(redis=r)
+    assert st["redis_ok"] is False