Fix issue label deletion with Actions tokens (#37013)

Use shared repo permission resolution for Actions task users in issue
label remove and clear paths, and add a regression test for deleting
issue labels with a Gitea Actions token.

This fixes issue label deletion when the request is authenticated with a
Gitea Actions token.
Fixes #37011 

The bug was that the delete path re-resolved repository permissions
using the normal user permission helper, which does not handle Actions
task users. As a result, `DELETE
/api/v1/repos/{owner}/{repo}/issues/{index}/labels/{id}` could return
`500` for Actions tokens even though label listing and label addition
worked.

---------

Co-authored-by: Codex <codex@openai.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

models/issues/pull_list.go

@@ -93,7 +93,7 @@ func CanMaintainerWriteToBranch(ctx context.Context, p access_model.Permission,
 			if err != nil {
 				continue
 			}
-			prPerm, err := access_model.GetUserRepoPermission(ctx, pr.BaseRepo, user)
+			prPerm, err := access_model.GetIndividualUserRepoPermission(ctx, pr.BaseRepo, user)
 			if err != nil {
 				continue
 			}