test(lua): cover challenge routes in start endpoint

Extend tests/lua/endpoints/test_start.py with a TestStartChallenge
class covering the new challenge branch. No new fixtures: challenges
start from MENU like the existing start tests, and the wrong-state case
reuses the existing start/state-BLIND_SELECT fixture. fixtures.json is
unchanged.

- happy_path (parametrized c_omelette_1, c_jokerless_1, c_mad_world_1):
  lands in BLIND_SELECT with deck=b_challenge, stake=stake_white, and
  the challenge id echoed in the gamestate.
- with_seed: proves challenge composes freely with seed.
- effect_applied (deep): The Omelette starts with 5 Eggs — asserts the
  challenge actually took effect, not just the flag.
- conflict_with_deck / _stake / _both: BAD_REQUEST "cannot be combined".
- invalid_id: BAD_REQUEST "Expected a c_* challenge id".
- wrong_type: schema-level BAD_REQUEST "must be of type string".

The existing TestStartEndpoint / validation / state-requirement tests
stay green unchanged — they are the regression guard for the normal
path (deck/stake are now schema-optional, enforced in execute with the
same verbatim error strings).

Author: S1M0N38

tests/lua/endpoints/test_start.py

@@ -164,3 +164,107 @@ def test_start_from_BLIND_SELECT(self, client: httpx.Client):
             "INVALID_STATE",
             "Method 'start' requires one of these states: MENU",
         )
+
+
+class TestStartChallenge:
+    """Tests for the challenge-run branch of the start endpoint.
+
+    Challenges are Balatro's 20 fixed-preset runs. The `challenge` param is
+    mutually exclusive with `deck`/`stake` but composes freely with `seed`.
+    """
+
+    @pytest.mark.parametrize(
+        "challenge_id",
+        [
+            "c_omelette_1",
+            "c_jokerless_1",
+            "c_mad_world_1",
+        ],
+    )
+    def test_start_challenge_happy_path(self, client: httpx.Client, challenge_id: str):
+        """A challenge run lands in BLIND_SELECT under the b_challenge deck."""
+        response = api(client, "menu", {})
+        assert_gamestate_response(response, state="MENU")
+        response = api(client, "start", {"challenge": challenge_id})
+        assert_gamestate_response(
+            response,
+            state="BLIND_SELECT",
+            challenge=challenge_id,
+            deck="b_challenge",
+            stake="stake_white",
+        )
+
+    def test_start_challenge_with_seed(self, client: httpx.Client):
+        """challenge composes freely with seed."""
+        response = api(client, "menu", {})
+        assert_gamestate_response(response, state="MENU")
+        response = api(
+            client, "start", {"challenge": "c_omelette_1", "seed": "TEST123"}
+        )
+        assert_gamestate_response(
+            response,
+            state="BLIND_SELECT",
+            challenge="c_omelette_1",
+            seed="TEST123",
+        )
+
+    def test_start_challenge_effect_applied(self, client: httpx.Client):
+        """Deep check: The Omelette starts with 5 Eggs — proves the challenge
+        actually took effect, not just the challenge flag."""
+        response = api(client, "menu", {})
+        assert_gamestate_response(response, state="MENU")
+        response = api(client, "start", {"challenge": "c_omelette_1"})
+        gamestate = assert_gamestate_response(response, state="BLIND_SELECT")
+        assert gamestate["jokers"]["count"] == 5
+        assert {c["key"] for c in gamestate["jokers"]["cards"]} == {"j_egg"}
+
+    def test_start_challenge_conflict_with_deck(self, client: httpx.Client):
+        """challenge cannot be combined with deck."""
+        response = api(client, "menu", {})
+        assert_gamestate_response(response, state="MENU")
+        response = api(
+            client,
+            "start",
+            {"challenge": "c_omelette_1", "deck": "b_red"},
+        )
+        assert_error_response(response, "BAD_REQUEST", "cannot be combined")
+
+    def test_start_challenge_conflict_with_stake(self, client: httpx.Client):
+        """challenge cannot be combined with stake."""
+        response = api(client, "menu", {})
+        assert_gamestate_response(response, state="MENU")
+        response = api(
+            client,
+            "start",
+            {"challenge": "c_omelette_1", "stake": "stake_white"},
+        )
+        assert_error_response(response, "BAD_REQUEST", "cannot be combined")
+
+    def test_start_challenge_conflict_with_both(self, client: httpx.Client):
+        """challenge cannot be combined with deck and stake together."""
+        response = api(client, "menu", {})
+        assert_gamestate_response(response, state="MENU")
+        response = api(
+            client,
+            "start",
+            {
+                "challenge": "c_omelette_1",
+                "deck": "b_red",
+                "stake": "stake_white",
+            },
+        )
+        assert_error_response(response, "BAD_REQUEST", "cannot be combined")
+
+    def test_start_challenge_invalid_id(self, client: httpx.Client):
+        """An unknown challenge id is rejected against live G.CHALLENGES."""
+        response = api(client, "menu", {})
+        assert_gamestate_response(response, state="MENU")
+        response = api(client, "start", {"challenge": "c_nope_1"})
+        assert_error_response(response, "BAD_REQUEST", "Expected a c_* challenge id")
+
+    def test_start_challenge_wrong_type(self, client: httpx.Client):
+        """challenge must be a string (schema-level type check)."""
+        response = api(client, "menu", {})
+        assert_gamestate_response(response, state="MENU")
+        response = api(client, "start", {"challenge": 123})
+        assert_error_response(response, "BAD_REQUEST", "must be of type string")