Fix import of LFS files in GitHub import

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/48512 may have
exposed a bug in GitHub LFS import that caused LFS objects to be
downloaded sequentially via `LfsImportService` rather than in
parallel. Instead of returning individual `LfsObject` objects, the
service would import all files and return a Hash indicating success or
failure. This error was caught by an exception handler with the message:

```
undefined method `oid' for [:status, :success]:Array
```

This caught exception prevented the failure from stalling out the
importer process. We may want to reintroduce that behavior.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/289669

changelogs/unreleased/sh-fix-github-lfs-import.yml

+---
+title: Fix import of LFS files in GitHub import
+merge_request: 48722
+author:
+type: fixed

lib/gitlab/github_import/importer/lfs_objects_importer.rb

@@ -23,7 +23,7 @@ module Gitlab
         end
 
         def each_object_to_import
-          lfs_objects = Projects::LfsPointers::LfsImportService.new(project).execute
+          lfs_objects = Projects::LfsPointers::LfsObjectDownloadListService.new(project).execute
 
           lfs_objects.each do |object|
             yield object

spec/lib/gitlab/github_import/importer/lfs_objects_importer_spec.rb

@@ -56,9 +56,9 @@ RSpec.describe Gitlab::GithubImport::Importer::LfsObjectsImporter do
       importer = described_class.new(project, client, parallel: false)
       lfs_object_importer = double(:lfs_object_importer)
 
-      allow(importer)
-        .to receive(:each_object_to_import)
-        .and_yield(lfs_download_object)
+      expect_next_instance_of(Projects::LfsPointers::LfsObjectDownloadListService) do |service|
+        expect(service).to receive(:execute).and_return([lfs_download_object])
+      end
 
       expect(Gitlab::GithubImport::Importer::LfsObjectImporter)
         .to receive(:new)
@@ -79,9 +79,9 @@ RSpec.describe Gitlab::GithubImport::Importer::LfsObjectsImporter do
     it 'imports each lfs object in parallel' do
       importer = described_class.new(project, client)
 
-      allow(importer)
-        .to receive(:each_object_to_import)
-        .and_yield(lfs_download_object)
+      expect_next_instance_of(Projects::LfsPointers::LfsObjectDownloadListService) do |service|
+        expect(service).to receive(:execute).and_return([lfs_download_object])
+      end
 
       expect(Gitlab::GithubImport::ImportLfsObjectWorker)
         .to receive(:perform_async)