fix(covenantsigner): correctly distinguish single vs both unparseable timestamps

The earlier tiebreaker fix incorrectly entered the "both unparseable"
branch when only the candidate had an invalid timestamp. Parse both
timestamps explicitly to distinguish three cases: only candidate bad
(keep existing), only existing bad (replace), both bad (lexicographic
tiebreak).

Author: piotr-roslaniec

pkg/covenantsigner/store.go

@@ -260,14 +260,38 @@ func (s *Store) load() error {
 						// candidate's timestamp is valid, the failure is on
 						// the existing job -- replace it. Otherwise skip the
 						// candidate.
-						if _, parseErr := time.Parse(time.RFC3339Nano, job.UpdatedAt); parseErr != nil {
+						_, existingParseErr := time.Parse(time.RFC3339Nano, existing.UpdatedAt)
+						_, candidateParseErr := time.Parse(time.RFC3339Nano, job.UpdatedAt)
+
+						switch {
+						case candidateParseErr != nil && existingParseErr == nil:
+							// Only the candidate is unparseable; keep existing.
+							logger.Warnf(
+								"skipping job [%s] with invalid timestamp on duplicate route key [%s/%s] (keeping [%s]): [%v]",
+								job.RequestID,
+								job.Route,
+								job.RouteRequestID,
+								existing.RequestID,
+								err,
+							)
+							continue
+						case candidateParseErr == nil && existingParseErr != nil:
+							// Only the existing is unparseable; replace with candidate.
+							logger.Warnf(
+								"replacing job [%s] with invalid timestamp on duplicate route key [%s/%s]: [%v]",
+								existing.RequestID,
+								job.Route,
+								job.RouteRequestID,
+								err,
+							)
+						default:
 							// Both timestamps are unparseable. Use
 							// lexicographic RequestID as a deterministic
-							// tiebreaker so the outcome does not depend on
-							// file iteration order.
+							// tiebreaker so the outcome does not depend
+							// on file iteration order.
 							if existing.RequestID <= job.RequestID {
 								logger.Warnf(
-									"skipping job [%s] with invalid timestamp on duplicate route key [%s/%s] (keeping [%s]): [%v]",
+									"skipping job [%s] on duplicate route key [%s/%s] (keeping [%s], lexicographic tiebreak): [%v]",
 									job.RequestID,
 									job.Route,
 									job.RouteRequestID,
@@ -277,15 +301,7 @@ func (s *Store) load() error {
 								continue
 							}
 							logger.Warnf(
-								"replacing job [%s] with invalid timestamp on duplicate route key [%s/%s] (both unparseable, lexicographic tiebreak): [%v]",
-								existing.RequestID,
-								job.Route,
-								job.RouteRequestID,
-								err,
-							)
-						} else {
-							logger.Warnf(
-								"replacing job [%s] with invalid timestamp on duplicate route key [%s/%s]: [%v]",
+								"replacing job [%s] on duplicate route key [%s/%s] (lexicographic tiebreak): [%v]",
 								existing.RequestID,
 								job.Route,
 								job.RouteRequestID,