fix: address pre-push review findings

tests/test_reliability_persistence.py

@@ -87,6 +87,28 @@ def test_merge_observation_records_metadata_for_fresh_value():
     assert merged["last_error"] is None
 
 
+def test_merge_observation_rejects_error_only_payload_as_fresh_data():
+    old = {
+        "value": 1.25,
+        "observed_at": "2026-07-25T12:00:00+00:00",
+        "source": "lookintobitcoin",
+        "stale": False,
+        "last_error": None,
+    }
+
+    merged = merge_observation(
+        old,
+        {"value": None, "error": "No data returned"},
+        source="lookintobitcoin",
+        error="metric missing from scrape",
+    )
+
+    assert merged["value"] == 1.25
+    assert merged["observed_at"] == old["observed_at"]
+    assert merged["stale"] is True
+    assert merged["last_error"] == "No data returned"
+
+
 @pytest.mark.parametrize("timestamp", [None, "", "not-a-time"])
 def test_onchain_refresh_due_when_timestamp_is_missing_or_invalid(timestamp):
     assert onchain_refresh_due(timestamp, now=datetime(2026, 7, 26, tzinfo=timezone.utc)) is True