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/issue_xref.go

@@ -207,7 +207,7 @@ func (issue *Issue) verifyReferencedIssue(stdCtx context.Context, ctx *crossRefe
 
 	// Check doer permissions; set action to None if the doer can't change the destination
 	if refIssue.RepoID != ctx.OrigIssue.RepoID || ref.Action != references.XRefActionNone {
-		perm, err := access_model.GetUserRepoPermission(stdCtx, refIssue.Repo, ctx.Doer)
+		perm, err := access_model.GetDoerRepoPermission(stdCtx, refIssue.Repo, ctx.Doer)
 		if err != nil {
 			return nil, references.XRefActionNone, err
 		}