test(sp41): expand parametric coverage to 20 visits (10 batch-1 + 10 batch-2)

The fixture-driven test now exercises both spot-check batches:
- spot_check_visits.csv        (10 NOT_IN_835 visits)
- spot_check_visits_batch2.csv (10 DENIED+PARTIAL visits)

All 20 visits run through build_claim_output → serialize_837 → SP40
local validator (25 R-codes) → structural_spot_check. Full backend
suite: 1568 passed (was 1567).

backend/tests/test_rebill_spot_check.py

@@ -380,7 +380,13 @@ def _load_visit_rows_from_csv(path) -> list[VisitRow]:
     return out
 
 
-@pytest.mark.parametrize("fixture_csv", ["tests/fixtures/spot_check_visits.csv"])
+@pytest.mark.parametrize(
+    "fixture_csv",
+    [
+        "tests/fixtures/spot_check_visits.csv",       # 10 batch-1 (NOT_IN_835)
+        "tests/fixtures/spot_check_visits_batch2.csv", # 10 batch-2 (DENIED+PARTIAL)
+    ],
+)
 def test_every_visit_in_fixture_passes_full_pipeline(fixture_csv):
     """For each visit in the fixture CSV, the full pipeline (build → serialize →
     SP40 local validator) passes.