fix unit tests

Signed-off-by: Xiaoxuan Wang <[email protected]>

cmd/oras/internal/option/packer.go

@@ -39,7 +39,7 @@ const (
 
 var (
 	errAnnotationConflict    = errors.New("`--annotation` and `--annotation-file` cannot be both specified")
-	errAnnotationFormat      = errors.New("missing key in `--annotation` flag")
+	errAnnotationFormat      = errors.New("annotation value doesn't match the required format")
 	errAnnotationDuplication = errors.New("duplicate annotation key")
 	errPathValidation        = errors.New("absolute file path detected. If it's intentional, use --disable-path-validation flag to skip this check")
 )
@@ -112,10 +112,7 @@ func (opts *Packer) LoadManifestAnnotations() (annotations map[string]map[string
 	if len(opts.ManifestAnnotations) != 0 {
 		annotations = make(map[string]map[string]string)
 		if err = parseAnnotationFlags(opts.ManifestAnnotations, annotations); err != nil {
-			return nil, &oerrors.Error{
-				Err:            errors.New("annotation value doesn't match the required format"),
-				Recommendation: `Please use the correct format in the flag: --annotation "key=value"`,
-			}
+			return nil, err
 		}
 	}
 	return
@@ -137,7 +134,10 @@ func parseAnnotationFlags(flags []string, annotations map[string]map[string]stri
 	for _, anno := range flags {
 		key, val, success := strings.Cut(anno, "=")
 		if !success {
-			return fmt.Errorf("%w: %s", errAnnotationFormat, anno)
+			return &oerrors.Error{
+				Err:            fmt.Errorf("%w", errAnnotationFormat),
+				Recommendation: `Please use the correct format in the flag: --annotation "key=value"`,
+			}
 		}
 		if _, ok := manifestAnnotations[key]; ok {
 			return fmt.Errorf("%w: %v, ", errAnnotationDuplication, key)