chore: import upstream snapshot with attribution

tests/test_corrupt_graph_json.py

@@ -0,0 +1,53 @@
+"""Corrupt graph.json produces an actionable error, not a raw traceback (#1536/#1537).
+
+Three load paths call json.loads on graph.json — build_merge (`--update`),
+affected.load_graph (`graphify prs`), and diagnostics._read_json_file
+(`graphify diagnose`). A truncated / invalid file (incomplete write, power loss,
+manual edit) must raise a clear RuntimeError with recovery guidance at each.
+"""
+from __future__ import annotations
+
+import pytest
+
+from graphify.build import build_merge
+from graphify.affected import load_graph
+from graphify.diagnostics import _read_json_file
+
+_CORRUPT = '{"nodes": [{"id": "a", "labe'   # truncated mid-object
+
+
+def _corrupt(tmp_path):
+    p = tmp_path / "graph.json"
+    p.write_text(_CORRUPT, encoding="utf-8")
+    return p
+
+
+def test_build_merge_corrupt_graph_raises_runtimeerror(tmp_path):
+    p = _corrupt(tmp_path)
+    with pytest.raises(RuntimeError, match=r"Cannot read .*incremental merge|rebuild"):
+        build_merge([], graph_path=p, dedup=False)
+
+
+def test_affected_load_graph_corrupt_raises_runtimeerror(tmp_path):
+    p = _corrupt(tmp_path)
+    with pytest.raises(RuntimeError, match=r"Cannot read graph file|regenerate"):
+        load_graph(p)
+
+
+def test_diagnostics_read_corrupt_raises_runtimeerror(tmp_path):
+    p = _corrupt(tmp_path)
+    with pytest.raises(RuntimeError, match=r"Cannot parse|corrupted"):
+        _read_json_file(p)
+
+
+def test_valid_graph_still_loads(tmp_path):
+    """Happy path unchanged: a well-formed graph.json loads without raising."""
+    p = tmp_path / "graph.json"
+    p.write_text(
+        '{"nodes": [{"id": "a", "label": "a", "file_type": "code"}], "edges": []}',
+        encoding="utf-8",
+    )
+    # none of these should raise
+    load_graph(p)
+    _read_json_file(p)
+    build_merge([], graph_path=p, dedup=False)