fix(isolation): distinguish accident links from same-numbered nodes

Prefer link endpoints for legacy accident IDs and allow explicit node selection. Read authoritative initial statuses and sources, verify source isolation, and separate direct isolation from newly disconnected supply nodes. Return validation errors as HTTP 422.

The node-first lookup misrouted adjacent Fengyang pipes 19025 and 19026 into unrelated regions. Regression cases fail against the previous algorithm and pass after this change.

tests/unit/test_wndb_network_views.py

@@ -52,6 +52,36 @@ def test_network_link_nodes_use_one_unified_view_query(monkeypatch) -> None:
     assert "FROM gis.network_links" in calls[0][1]
 
 
+def test_isolation_snapshot_reads_authoritative_links_statuses_and_sources_once(monkeypatch):
+    calls = []
+
+    def fake_read_all(name, statement, params=None):
+        calls.append((name, statement, params))
+        return [
+            {"id": "P", "element_type": "pipe", "start_node_id": "A", "end_node_id": "B", "status": None},
+            {"id": "P-closed", "element_type": "pipe", "start_node_id": "A", "end_node_id": "R", "status": "CLOSED"},
+            {"id": "V", "element_type": "valve", "start_node_id": "B", "end_node_id": "R", "status": "CLOSED"},
+            {"id": "PU", "element_type": "pump", "start_node_id": "T", "end_node_id": "A", "status": "CLOSED"},
+            {"id": "R", "element_type": "reservoir", "start_node_id": None, "end_node_id": None, "status": None},
+            {"id": "T", "element_type": "tank", "start_node_id": None, "end_node_id": None, "status": None},
+        ]
+
+    monkeypatch.setattr(network_views, "read_all", fake_read_all)
+
+    assert network_views.get_valve_isolation_topology("project_a") == (
+        ["P:pipe:A:B", "P-closed:pipe:A:R", "V:valve:B:R", "PU:pump:T:A"],
+        ["R", "T"], ["P-closed", "V", "PU"],
+    )
+    assert len(calls) == 1
+    name, statement, _params = calls[0]
+    assert name == "project_a"
+    assert "FROM network.links" in statement
+    assert "network.link_initial_settings" in statement
+    assert "COALESCE(s.status::text, p.status::text)" in statement
+    assert "FROM network.nodes" in statement
+    assert "UNION ALL" in statement
+
+
 def test_topology_rows_are_loaded_in_two_batch_queries(monkeypatch) -> None:
     calls = []
     responses = [