chore: clean code of json metadata output for attach

Signed-off-by: Billy Zha <[email protected]>

cmd/oras/internal/display/metadata/json/attach.go

@@ -27,9 +27,9 @@ import (
 
 // AttachHandler handles json metadata output for attach events.
 type AttachHandler struct {
-	out    io.Writer
-	target *option.Target
-	root   ocispec.Descriptor
+	out  io.Writer
+	path string
+	root ocispec.Descriptor
 }
 
 // NewAttachHandler creates a new handler for attach events.
@@ -41,11 +41,11 @@ func NewAttachHandler(out io.Writer) metadata.AttachHandler {
 
 // OnAttached implements AttachHandler.
 func (ah *AttachHandler) OnAttached(target *option.Target, root ocispec.Descriptor, _ ocispec.Descriptor) {
-	ah.target = target
+	ah.path = target.Path
 	ah.root = root
 }
 
 // Render is called when the attach command is completed.
 func (ah *AttachHandler) Render() error {
-	return output.PrintPrettyJSON(ah.out, model.NewAttach(ah.root, ah.target.Path))
+	return output.PrintPrettyJSON(ah.out, model.NewAttach(ah.root, ah.path))
 }