Add cross-language wire-format fixture test and wrong-shape sync coverage

The Python and Dart test suites each only verified round-tripping through
their own encoder/decoder, so a Record.to_dict()/toJson() key mismatch
between the two (deleted_hlc vs deletedHlc) would have gone undetected
despite being the exact thing this shared library exists to prevent. Pin
one canonical fixture in both suites so a future divergence fails loudly
on at least one side. Also add a test for a valid-JSON-but-wrong-shape
remote push, exercising the full (ValueError, KeyError, TypeError) skip
path, not just JSON syntax errors.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017McuA1nAi6jBT1EaS5BrVf

crdt-sync/crdt_sync/tests/test_record.py

@@ -109,3 +109,32 @@ class TestRecordDictRoundTrip:
     def test_round_trips_a_record_with_no_delete_clock(self) -> None:
         record = Record(id="x", fields={}, deleted=False, deleted_hlc=None)
         assert Record.from_dict(record.to_dict()) == record
+
+
+class TestCrossLanguageWireFormat:
+    """Pins the exact wire shape shared with crdt_sync_dart's Record.
+
+    Values chosen here are duplicated verbatim in crdt_sync_dart's
+    `test/record_test.dart` (see its ``TestCrossLanguageWireFormat``
+    group). If this test and that one both pass, the two languages agree
+    on the wire format; if only one changes, the two suites diverge and
+    at least one of them fails -- catching exactly the kind of
+    key-naming mismatch (``deleted_hlc`` vs ``deletedHlc``) that neither
+    language's own round-trip test can see on its own.
+    """
+
+    def test_matches_the_fixture_shared_with_the_dart_package(self) -> None:
+        record = Record(
+            id="abc123",
+            fields={"text": ("hello", Hlc(wall_time_ms=100, counter=0, node_id="pc"))},
+            deleted=True,
+            deleted_hlc=Hlc(wall_time_ms=1000, counter=0, node_id="node-a"),
+        )
+        expected = {
+            "id": "abc123",
+            "fields": {"text": ["hello", "1970-01-01T00:00:00.100Z-0000-pc"]},
+            "deleted": True,
+            "deleted_hlc": "1970-01-01T00:00:01.000Z-0000-node-a",
+        }
+        assert record.to_dict() == expected
+        assert Record.from_dict(expected) == record

crdt-sync/crdt_sync/tests/test_sync.py

@@ -117,6 +117,28 @@ class TestSyncLog:
         assert merged == {}
         assert "Unparsable log" in caplog.text
 
+    def test_skips_a_device_whose_pushed_json_has_the_wrong_shape(self) -> None:
+        """Valid JSON that isn't a record map (e.g. from an incompatible
+        writer) must be skipped like corrupt JSON, not crash the whole
+        sync -- this is what the broad ``(ValueError, KeyError, TypeError)``
+        catch in ``_pull_remote_logs`` is for, not just JSON syntax errors.
+        """
+        client = _mock_client(
+            devices=("phone",),
+            files={"devices/phone/log.json": '{"a": 5}'},
+        )
+
+        merged = sync_log(
+            client=client,
+            device_id="pc",
+            path_prefix="devices",
+            local_log={},
+            encode=_encode,
+            decode=_decode,
+        )
+
+        assert merged == {}
+
     def test_merges_in_a_remote_devices_entries(
         self, make_hlc: Callable[..., Hlc]
     ) -> None: