fix(api): tolerate legacy incident decision chains

apps/api/tests/test_incident_service_resolve_idempotency.py

@@ -12,13 +12,18 @@ test_incident_service_resolve_idempotency
 重新放大「resolve_incident 重複觸發 postmortem 洗版」的舊風險。
 """
 
+from datetime import UTC, datetime
 from types import SimpleNamespace
 from unittest.mock import AsyncMock
 
 import pytest
 
 from src.models.incident import IncidentStatus
-from src.services.incident_service import IncidentService, normalize_status
+from src.services.incident_service import (
+    IncidentService,
+    normalize_status,
+    parse_decision_chain,
+)
 
 
 @pytest.mark.asyncio
@@ -95,3 +100,37 @@ def test_normalize_status_accepts_db_enum_name() -> None:
     """PostgreSQL SQLEnum 會存 Enum name；讀回時必須正規化成 Pydantic value。"""
     assert normalize_status("INVESTIGATING") == "investigating"
     assert normalize_status(IncidentStatus.CLOSED) == "closed"
+
+
+def test_parse_decision_chain_skips_legacy_list_payload() -> None:
+    """舊資料 decision_chain 可能是 list；不應阻斷 incident hydrate / resolve。"""
+    assert parse_decision_chain([{"stage": "router"}], "INC-LEGACY-CHAIN") is None
+
+
+def test_record_to_incident_tolerates_legacy_decision_chain_list() -> None:
+    """DB fallback 必須能讀回舊 incident，即使 decision_chain 不是新 schema。"""
+    now = datetime.now(UTC)
+    record = SimpleNamespace(
+        incident_id="INC-LEGACY-CHAIN",
+        status="INVESTIGATING",
+        severity="P2",
+        signals=[],
+        affected_services=[],
+        decision_chain=[{"stage": "router"}],
+        proposal_ids=[],
+        outcome=None,
+        created_at=now,
+        updated_at=now,
+        resolved_at=None,
+        closed_at=None,
+        ttl_days=7,
+        vectorized=False,
+        notification_type="TYPE-3",
+        alert_category="host_resource",
+    )
+
+    incident = IncidentService()._record_to_incident(record)
+
+    assert incident.incident_id == "INC-LEGACY-CHAIN"
+    assert incident.status == IncidentStatus.INVESTIGATING
+    assert incident.decision_chain is None