test(api)+docs: assert matchedRemittance=null; fail-loud on fixture drift; tighten docstring

backend/tests/test_api_claim_detail.py

@@ -103,6 +103,11 @@ def test_get_claim_detail_happy_path(seeded_store):
         "validation", "rawSegments", "matchedRemittance", "stateHistory",
     ):
         assert k in body, f"missing key {k!r}"
+    # Spec §3.2: matchedRemittance is `null` for an unpaired claim
+    # (the dominant UI render path). The matched-populated case is
+    # covered separately in
+    # test_get_claim_detail_matched_remit_inlines_summary.
+    assert body["matchedRemittance"] is None
     assert body["id"] == cid
     # billedAmount is a number (not a Decimal string).
     assert isinstance(body["billedAmount"], (int, float))
@@ -140,7 +145,11 @@ def test_get_claim_detail_state_history_includes_manual_match(seeded_store_with_
         "/api/reconciliation/unmatched", headers=JSON,
     ).json()
     if not unmatched["claims"] or not unmatched["remittances"]:
-        pytest.skip("seeded fixtures don't produce an unmatched pair")
+        pytest.fail(
+            "co_medicaid_837p.txt + co_medicaid_835.txt fixtures no longer "
+            "produce an unmatched pair; matched-remit tests need updating "
+            "(see seeded_store_with_pair docstring for the expected invariant)"
+        )
     claim_id = unmatched["claims"][0]["id"]
     remit_id = unmatched["remittances"][0]["id"]
 
@@ -180,7 +189,11 @@ def test_get_claim_detail_matched_remit_inlines_summary(seeded_store_with_pair):
         "/api/reconciliation/unmatched", headers=JSON,
     ).json()
     if not unmatched["claims"] or not unmatched["remittances"]:
-        pytest.skip("seeded fixtures don't produce an unmatched pair")
+        pytest.fail(
+            "co_medicaid_837p.txt + co_medicaid_835.txt fixtures no longer "
+            "produce an unmatched pair; matched-remit tests need updating "
+            "(see seeded_store_with_pair docstring for the expected invariant)"
+        )
     claim_id = unmatched["claims"][0]["id"]
     remit_id = unmatched["remittances"][0]["id"]
     seeded_store_with_pair.post(