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>

routers/api/v1/repo/repo.go

@@ -221,7 +221,7 @@ func Search(ctx *context.APIContext) {
 			})
 			return
 		}
-		permission, err := access_model.GetUserRepoPermission(ctx, repo, ctx.Doer)
+		permission, err := access_model.GetDoerRepoPermission(ctx, repo, ctx.Doer)
 		if err != nil {
 			ctx.JSON(http.StatusInternalServerError, api.SearchError{
 				OK:    false,
@@ -588,7 +588,7 @@ func GetByID(ctx *context.APIContext) {
 		return
 	}
 
-	permission, err := access_model.GetUserRepoPermission(ctx, repo, ctx.Doer)
+	permission, err := access_model.GetDoerRepoPermission(ctx, repo, ctx.Doer)
 	if err != nil {
 		ctx.APIErrorInternal(err)
 		return