Keep malformed KTX seat payloads actionable

Convert malformed Korail seat-detail payloads into the existing CLI failure path so advisory lookup callers get a clear retryable error instead of an AttributeError.

Constraint: PR #295 review watch item identified successful-but-malformed seat_infos payloads as an unhelpful crash surface.

Rejected: Letting raw adapter fallbacks leak AttributeError | CLI users need actionable SystemExit diagnostics at the command boundary.

Confidence: high

Scope-risk: narrow

Directive: Keep detailed seat lookup advisory; validate raw Korail shapes before exposing fields to JSON callers.

Tested: PYTHONPATH=.:scripts python3 -m unittest scripts.test_ktx_booking; node --test scripts/skill-docs.test.js; npm run typecheck; python3 -m compileall -q scripts/ktx_booking.py scripts/test_ktx_booking.py; ruff check scripts/ktx_booking.py scripts/test_ktx_booking.py; shellcheck scripts/validate-skills.sh; bash scripts/validate-skills.sh; PATH=<pyenv 3.11.9 shim> npm run ci

Not-tested: Plain npm run ci with /opt/homebrew Python 3.14 due local pyexpat/libexpat linkage error reproduced before project tests.

scripts/ktx_booking.py

@@ -927,9 +927,20 @@ def command_seats(args: argparse.Namespace) -> None:
     car_payloads: list[dict[str, object]] = []
     for car in cars:
         raw = client.car_seats(raw_train, str(car["car_no_raw"]), passenger_count, room_class)
-        raw_seats = raw.get("seat_infos", {}).get("seat_info", [])
+        seat_infos = raw.get("seat_infos") if isinstance(raw, dict) else None
+        if not isinstance(seat_infos, dict):
+            raise SystemExit(
+                f"seat detail data is unavailable for car_no {car['car_no']}; "
+                "retry search or choose another train"
+            )
+        raw_seats = seat_infos.get("seat_info", [])
         if isinstance(raw_seats, dict):
             raw_seats = [raw_seats]
+        if not isinstance(raw_seats, list):
+            raise SystemExit(
+                f"seat detail data is unavailable for car_no {car['car_no']}; "
+                "retry search or choose another train"
+            )
         all_seats = [normalize_seat(seat) for seat in raw_seats if seat.get("h_con_seat_no") != "0A"]
         seats = sort_seats_for_booking(all_seats)
         if args.available_only:

scripts/test_ktx_booking.py

@@ -797,6 +797,40 @@ class KtxBookingTests(unittest.TestCase):
 
         self.assertIn("car_no 5", str(exc.exception))
 
+    def test_command_seats_fails_when_seat_payload_is_malformed(self):
+        selected = FakeTrain(train_no="009", dep_time="090000", arr_time="113000", label="selected")
+        train_id = ktx_booking.normalize_train(selected, index=1)["train_id"]
+        client = FakeClient(
+            [],
+            train_details=[(selected, {"h_trn_no": "009"})],
+            cars=[{"h_srcar_no": "05", "h_psrm_cl_cd": "1", "h_seat_cnt": "48", "h_rest_seat_cnt": "4"}],
+        )
+        client.car_seats = lambda *args, **kwargs: {"seat_infos": None}
+        args = argparse.Namespace(
+            dep="서울",
+            arr="부산",
+            date="20260328",
+            time="090000",
+            adults=1,
+            children=0,
+            toddlers=0,
+            seniors=0,
+            train_id=train_id,
+            room="general",
+            train_type="ktx",
+            car_no=None,
+            available_only=False,
+            power_only=False,
+            limit=10,
+        )
+
+        with patch.object(ktx_booking, "build_client", return_value=client):
+            with self.assertRaises(SystemExit) as exc:
+                with redirect_stdout(io.StringIO()):
+                    ktx_booking.command_seats(args)
+
+        self.assertIn("seat detail data is unavailable", str(exc.exception))
+
     def test_command_seats_fails_when_car_data_is_unavailable(self):
         selected = FakeTrain(train_no="009", dep_time="090000", arr_time="113000", label="selected")
         train_id = ktx_booking.normalize_train(selected, index=1)["train_id"]