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/web/repo/compare.go

@@ -226,9 +226,9 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 	// If we're not merging from the same repo:
 	if !isSameRepo {
 		// Assert ctx.Doer has permission to read headRepo's codes
-		permHead, err := access_model.GetUserRepoPermission(ctx, headRepo, ctx.Doer)
+		permHead, err := access_model.GetDoerRepoPermission(ctx, headRepo, ctx.Doer)
 		if err != nil {
-			ctx.ServerError("GetUserRepoPermission", err)
+			ctx.ServerError("GetDoerRepoPermission", err)
 			return nil
 		}
 		if !permHead.CanRead(unit.TypeCode) {