test: lock that the deck list omits cards

PR #30 split the Deck response contract (light Deck for lists,
DeckWithCards for detail) but no test asserted that cards is absent
from list responses — the existing tests passed either way.

Add a test that creates a deck WITH a card and asserts the list item
has no cards key. Under the old single-schema design this would have
failed (it returned the misleading cards: []), so it pins the fix.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: lesnik512

tests/test_decks.py

@@ -35,6 +35,16 @@ async def test_get_decks(client: AsyncClient) -> None:
         assert v == getattr(deck, k)
 
 
+async def test_list_decks_omits_cards(client: AsyncClient) -> None:
+    deck = await factories.DeckModelFactory.create_async()
+    await factories.CardModelFactory.create_async(deck_id=deck.id)
+
+    response = await client.get("/api/decks/")
+    assert response.status_code == status_codes.HTTP_200_OK
+    item = response.json()["items"][0]
+    assert "cards" not in item
+
+
 async def test_get_one_deck(client: AsyncClient) -> None:
     deck = await factories.DeckModelFactory.create_async()
     card = await factories.CardModelFactory.create_async(deck_id=deck.id)