refactor improve NoBetterThan (#26126)

- The `NoBetterThan` function can only handle comparisons between
"pending," "success," "error," and "failure." For any other comparison,
we directly return false. This prevents logic errors like the one in
#26121.
- The callers of the `NoBetterThan` function should also avoid making
incomparable calls.

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Co-authored-by: yp05327 <576951401@qq.com>
Co-authored-by: puni9869 <80308335+puni9869@users.noreply.github.com>

services/convert/status.go

@@ -48,7 +48,7 @@ func ToCombinedStatus(ctx context.Context, statuses []*git_model.CommitStatus, r
 	retStatus.Statuses = make([]*api.CommitStatus, 0, len(statuses))
 	for _, status := range statuses {
 		retStatus.Statuses = append(retStatus.Statuses, ToCommitStatus(ctx, status))
-		if status.State.NoBetterThan(retStatus.State) {
+		if retStatus.State == "" || status.State.NoBetterThan(retStatus.State) {
 			retStatus.State = status.State
 		}
 	}