feat(ai_trade): poll_loop가 매 cycle 끝에 expired signal purge (F5 part 4)

Phase 5 consumer(agent-office /signal)가 안 붙은 상태에서도 state.signals가
무한 누적되지 않도록 매 cycle 끝에 state.purge_expired_signals(now) 호출.
expires_at < now인 signal 자동 제거.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ai_trade/tests/test_pull_worker.py

@@ -178,3 +178,64 @@ async def test_post_close_fires_at_1601_when_not_yet_today(monkeypatch):
     )
 
     assert post_close.await_count >= 1, "post-close가 16:01에 호출되지 않음 (F3 회귀)"
+
+
+async def test_poll_loop_purges_expired_signals(monkeypatch):
+    """F5 — 매 cycle 끝에 expired signal이 제거됨."""
+    from datetime import datetime as _dt
+    from zoneinfo import ZoneInfo as _ZI
+    import asyncio as _asyncio
+
+    from ai_trade import pull_worker
+    from ai_trade.state import PollState
+
+    _kst = _ZI("Asia/Seoul")
+    now = _dt(2026, 5, 18, 10, 0, tzinfo=_kst)
+
+    class FrozenDT:
+        @staticmethod
+        def now(tz=None):
+            return now
+
+    state = PollState()
+    state.signals = {
+        "OLD": {
+            "ticker": "OLD",
+            "expires_at": _dt(2026, 5, 18, 9, 0, tzinfo=_kst).isoformat(),
+            "cycle_id": 1,
+        },
+        "FRESH": {
+            "ticker": "FRESH",
+            "expires_at": _dt(2026, 5, 18, 10, 30, tzinfo=_kst).isoformat(),
+            "cycle_id": 1,
+        },
+    }
+
+    monkeypatch.setattr(pull_worker, "datetime", FrozenDT)
+    monkeypatch.setattr(pull_worker, "_is_market_day", lambda n: True)
+    monkeypatch.setattr(pull_worker, "_is_polling_window", lambda n: True)
+    monkeypatch.setattr(pull_worker, "_next_interval", lambda n: 0.01)
+    monkeypatch.setattr(pull_worker, "_run_polling_cycle", AsyncMock())
+    monkeypatch.setattr(pull_worker, "update_minute_momentum_for_all", lambda s: None)
+    monkeypatch.setattr(pull_worker, "_is_post_close_trigger", lambda *a, **k: False)
+
+    shutdown = _asyncio.Event()
+
+    async def stop_soon():
+        await _asyncio.sleep(0.05)
+        shutdown.set()
+
+    _asyncio.create_task(stop_soon())
+
+    await pull_worker.poll_loop(
+        client=MagicMock(),
+        state=state,
+        shutdown=shutdown,
+        kis_client=MagicMock(),
+        chronos=MagicMock(),
+        dedup=None,
+        settings=None,
+    )
+
+    assert "OLD" not in state.signals
+    assert "FRESH" in state.signals