fix: address pre-push review findings

dashboard/persistence.py

@@ -19,7 +19,7 @@ except ImportError:  # pragma: no cover - Windows fallback uses the process lock
 
 _LOCKS: dict[str, threading.RLock] = {}
 _LOCKS_GUARD = threading.Lock()
-_METADATA_KEYS = {"observed_at", "source", "stale", "last_error"}
+_METADATA_KEYS = {"observed_at", "source", "stale", "last_error", "error"}
 
 
 def _thread_lock(path: Path) -> threading.RLock:
@@ -165,7 +165,7 @@ def append_daily_jsonl(path: str | os.PathLike[str], entry: dict[str, Any]) -> b
     return True
 
 
-def _has_observation(payload: Any) -> bool:
+def has_observation(payload: Any) -> bool:
     if not isinstance(payload, dict):
         return payload is not None
     return any(value is not None for key, value in payload.items() if key not in _METADATA_KEYS)
@@ -180,7 +180,7 @@ def merge_observation(
     error: str | None = None,
 ) -> dict[str, Any]:
     """Annotate a fresh observation or retain the last-known-good value as stale."""
-    if _has_observation(observed):
+    if has_observation(observed):
         merged = dict(observed) if isinstance(observed, dict) else {"value": observed}
         merged.update(
             observed_at=observed_at or datetime.now(timezone.utc).isoformat(),
@@ -191,10 +191,11 @@ def merge_observation(
         return merged
 
     merged = dict(previous) if isinstance(previous, dict) else {}
+    observed_error = observed.get("error") if isinstance(observed, dict) else None
     merged.update(
         source=merged.get("source") or source,
         stale=True,
-        last_error=error or "metric was not observed",
+        last_error=observed_error or error or "metric was not observed",
     )
     merged.setdefault("observed_at", None)
     return merged