feat(investigation): preserve cumulative reference graphs

Accumulate investigation paths and preserve failed attempts across rounds. Keep graph annotations separate from execution state, synchronize the API contract, and consolidate documentation.

tests/unit/test_investigation_reference_states.py

@@ -0,0 +1,27 @@
+import pytest
+from pydantic import ValidationError
+
+from app.domain.schemas.operational_task import InvestigationDiagramNodeRef, InvestigationGraphUpdate
+
+
+@pytest.mark.parametrize("status", ["reference", "recorded", "needs_input", "failed", "not_taken"])
+def test_graph_updates_only_describe_reference_steps_and_recorded_outcomes(status):
+    update = InvestigationGraphUpdate.model_validate({"nodes": [{"node_id": "check", "label": "核查", "status": status}]})
+    assert update.nodes[0].status == status
+
+
+@pytest.mark.parametrize("legacy,expected", [
+    ("pending", "reference"), ("running", "reference"), ("completed", "recorded"),
+    ("waiting", "needs_input"), ("waiting_human", "needs_input"),
+    ("skipped", "not_taken"), ("cancelled", "not_taken"),
+])
+def test_old_diagrams_are_read_as_reference_annotations_without_changing_stored_documents(legacy, expected):
+    document = {"node_id": "check", "label": "核查", "status": legacy}
+    assert InvestigationDiagramNodeRef.model_validate(document).status == expected
+    assert document["status"] == legacy
+
+
+@pytest.mark.parametrize("status", ["running", "pending", "completed", "waiting", "waiting_human", "skipped", "cancelled"])
+def test_new_incremental_writes_reject_execution_lifecycle_states(status):
+    with pytest.raises(ValidationError):
+        InvestigationGraphUpdate.model_validate({"nodes": [{"node_id": "check", "label": "核查", "status": status}]})