fix(registry): accept OCI Image Index in plugin pull

Allow plugin pull to resolve tags that point to an OCI Image Index,
matching the fix already applied to chart pull in PR #31776.

Closes #32541

Signed-off-by: water <672684719@qq.com>

pkg/registry/plugin.go

@@ -62,6 +62,7 @@ func (c *Client) PullPlugin(ref string, pluginName string, options ...PluginPull
 	genericResult, err := genericClient.PullGeneric(ref, GenericPullOptions{
 		// Allow manifests and all layer types - we'll validate artifact type after download
 		AllowedMediaTypes: []string{
+			ocispec.MediaTypeImageIndex,
 			ocispec.MediaTypeImageManifest,
 			"application/vnd.oci.image.layer.v1.tar",
 			"application/vnd.oci.image.layer.v1.tar+gzip",