Add default_branch to KAS responses

KAS queries gitaly directly for agent configuration and GitOps.
Currently, KAS mimicks this default branch logic internally, and we
suspect it may sometimes get it wrong. So this may fix a bug, but it
will also allow us to remove duplicate logic from KAS.

See https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/issues/210

Changelog: changed

lib/api/internal/kubernetes.rb

@@ -39,6 +39,7 @@ module API
 
         def gitaly_repository(project)
           {
+            default_branch: project.default_branch_or_main,
             storage_name: project.repository_storage,
             relative_path: project.disk_path + '.git',
             gl_repository: repo_type.identifier_for_container(project),

spec/requests/api/internal/kubernetes_spec.rb

@@ -169,6 +169,7 @@ RSpec.describe API::Internal::Kubernetes do
               'features' => {}
             ),
             'gitaly_repository' => a_hash_including(
+              'default_branch' => project.default_branch_or_main,
               'storage_name' => project.repository_storage,
               'relative_path' => project.disk_path + '.git',
               'gl_repository' => "project-#{project.id}",