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_graph_migration.py

@@ -0,0 +1,45 @@
+from copy import deepcopy
+from datetime import datetime, timezone
+
+import pytest
+
+from scripts.migrate_investigation_graphs import prepare_progress_document
+from tests.support.operational_task_repository import _active_run, _context, _task_document
+
+
+@pytest.mark.parametrize("node_refs", [[], [
+    {"node_id": "old", "label": "旧尝试", "status": "failed", "detail": "原查询超时", "evidence_refs": ["e1"]},
+    {"node_id": "next", "label": "后续核查", "status": "running"},
+]])
+def test_migration_keeps_source_paths_failures_and_original_publication_provenance(node_refs):
+    now = datetime(2026, 11, 10, tzinfo=timezone.utc)
+    task = _task_document(_context(), now)
+    run = _active_run(task, now)
+    document = {
+        "revision": 7, "run_id": run["run_id"], "execution_generation": 1,
+        "source_session_id": "session-1", "published_at": now.isoformat(),
+        "progress": {"summary": "原调查说明", "diagram": {"title": "旧图", "node_refs": node_refs,
+            "mermaid": "flowchart LR\nold{旧尝试} -->|失败后重试| next[后续核查]"}},
+    }
+    original = deepcopy(document)
+    migrated = prepare_progress_document(document)
+    assert document == original
+    assert migrated["revision"] == 8
+    assert {key: migrated[key] for key in original if key not in {"revision", "progress"}} == {
+        key: original[key] for key in original if key not in {"revision", "progress"}
+    }
+    assert migrated["progress"]["summary"] == original["progress"]["summary"]
+    graph = migrated["progress"]["diagram"]
+    assert graph["mermaid"] == graph["source_mermaid"] == original["progress"]["diagram"]["mermaid"]
+    assert graph["edges"] == []
+    if node_refs:
+        assert graph["node_refs"][0]["detail"] == "原查询超时"
+        assert graph["node_refs"][0]["evidence_refs"] == ["e1"]
+        assert [node["status"] for node in graph["node_refs"]] == ["failed", "reference"]
+    assert prepare_progress_document(migrated) is None
+
+
+def test_migration_does_not_invent_a_graph_when_none_was_saved():
+    document = {"progress": {"summary": "仅说明", "diagram": None}}
+    assert prepare_progress_document(document) is None
+    assert document == {"progress": {"summary": "仅说明", "diagram": None}}