GitLab SCM Provider generator discards collected branches on a mid-pagination 404 which can prune Applications #29323

Checklist:

• I've searched in the docs and FAQ for my answer: https://bit.ly/argocd-faq.
• I've included steps to reproduce the bug.
• I've pasted the output of argocd version.

Describe the bug

GitlabProvider.listBranches treats 404 as no branches inside its pagination loop and returns an empty slice with a nil error that throws away every branch already collected from earlier pages.

for {
		gitlabBranches, resp, err := g.client.Branches.ListBranches(repo.RepositoryId, opt)
		// 404s are not an error here, just a normal false.
		if resp != nil && resp.StatusCode == http.StatusNotFound {
			return []gitlab.Branch{}, nil
		}
		if err != nil {
			return nil, err
		}
		for _, gitlabBranch := range gitlabBranches {
			branches = append(branches, *gitlabBranch)
		}

		if resp.NextPage == 0 {
			break
		}
		opt.Page = resp.NextPage
	}
	return branches, nil
}

This matters because the appset reconciler trusts nil error completely. A returned error is safe however, a successful empty result flows straight through to prune step so a repo with more than 100 branches that has 404s on page 2 can have all of its generated apps deleted rather than the generator failing and retrying. Interestingly, GitLab returns 404 rather than 403 for a project the token can't see so a token whose project access is revoked in the middle can hit this.

To Reproduce

This is a race against the GitLab API, so the deterministic reproduction is at the unit level.

1. Stand up an httptest server for /api/v4/projects/{id}/repository/branches that returns
   a full page of 100 branches with X-Next-Page: 2, then 404 for page=2.
2. Call listBranches with allBranches: true.
3. Observe it returns ([], nil) - the 100 branches from page 1 are gone, and there is no error.

Expected behavior

A 404 encountered while paginating should be returned as an error, so the reconciler records an ErrorOccurred condition and requeues with existing Applications untouched.

Screenshots

N/A

Version

argocd: v3.5.1+109ca7c
  BuildDate: 2026-08-12T11:51:48Z
  GitCommit: 109ca7ca71139e514114499d294a492e7910a965
  GitTreeState: clean
  GoVersion: go1.26.4
  Compiler: gc
  Platform: darwin/arm64
argocd-server: v3.5.1
  BuildDate: 2026-08-12T11:28:06Z
  GitCommit: 109ca7ca71139e514114499d294a492e7910a965
  GitTreeState: clean
  GitTag: v3.5.1
  GoVersion: go1.26.4
  Compiler: gc
  Platform: linux/arm64
  Kustomize Version: v5.8.1 2026-02-09T16:15:27Z
  Helm Version: v4.2.1+gd591a19
  Kubectl Version: v0.36.1
  Jsonnet Version: v0.22.0

Logs

Paste any relevant application logs here.