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>

services/contexttest/context_tests.go

@@ -125,7 +125,7 @@ func LoadRepo(t *testing.T, ctx gocontext.Context, repoID int64) {
 	repo.Owner, err = user_model.GetUserByID(ctx, repo.Repository.OwnerID)
 	assert.NoError(t, err)
 	repo.RepoLink = repo.Repository.Link()
-	repo.Permission, err = access_model.GetUserRepoPermission(ctx, repo.Repository, doer)
+	repo.Permission, err = access_model.GetDoerRepoPermission(ctx, repo.Repository, doer)
 	assert.NoError(t, err)
 }