fix(nwis): handle empty peaks response instead of raising KeyError

tests/nwis_test.py

@@ -332,7 +332,7 @@ def test_set_metadata_info_countyCd(self, httpx_mock):
 
 
 class TestReadRdb:
-    """Tests for the NWIS-specific _read_rdb wrapper.
+    """Tests for the NWIS-specific parse-then-format path.
 
     The format-agnostic parser is exercised in tests/rdb_test.py; this
     class pins the wrapper-specific contract — that an empty parser
@@ -374,6 +374,17 @@ def test_no_peaks_flows_through_format_response(self):
         assert isinstance(df, pd.DataFrame)
         assert df.empty
 
+    def test_malformed_peaks_frame_still_raises(self):
+        """Only an *empty* peaks frame is a legitimate empty result. A
+        non-empty frame with no ``peak_dt`` column is a malformed response --
+        a truncated or altered RDB header -- and must stay loud rather than be
+        returned silently without its datetime index.
+        """
+        df = pd.DataFrame({"peak_va": [1000]})
+
+        with pytest.raises(KeyError, match="peak_dt"):
+            format_response(df, service="peaks")
+
 
 class TestGetRecordDispatch:
     """``get_record`` is a router; each service must reach its own getter.