Ensure ci_pipelines.iid set before transaction in commit status API

Prior to this we were using `create!` which triggered the
`ensure_project_iid!` in a callback. This causes a
`CrossDatabaseModificationAcrossUnsupportedTablesError`. To avoid this
we will create the `iid` before saving. This is consistent with all
other places we create pipelines.

This change still doesn't allow us to remove
`spec/requests/api/commit_statuses_spec.rb` from the allowlist as there
are other calls to `project.ci_pipelines.create!` in this file.

We need a more holistic solution for that which is being discussed in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75316 .

lib/api/commit_statuses.rb

@@ -78,13 +78,16 @@ class CommitStatuses < ::API::Base
 
         name = params[:name] || params[:context] || 'default'
 
-        pipeline ||= user_project.ci_pipelines.create!(
+        pipeline ||= user_project.ci_pipelines.build(
           source: :external,
           sha: commit.sha,
           ref: ref,
           user: current_user,
           protected: user_project.protected_for?(ref))
 
+        pipeline.ensure_project_iid!
+        pipeline.save!
+
         authorize! :update_pipeline, pipeline
 
         status = GenericCommitStatus.running_or_pending.find_or_initialize_by(

spec/requests/api/commit_statuses_spec.rb

@@ -131,7 +131,7 @@ def create_status(commit, opts = {})
         %w[pending running success failed canceled].each do |status|
           context "for #{status}" do
             context 'when pipeline for sha does not exists' do
-              it 'creates commit status' do
+              it 'creates commit status and sets pipeline iid' do
                 post api(post_url, developer), params: { state: status }
 
                 expect(response).to have_gitlab_http_status(:created)
@@ -145,6 +145,8 @@ def create_status(commit, opts = {})
                 if status == 'failed'
                   expect(CommitStatus.find(json_response['id'])).to be_api_failure
                 end
+
+                expect(::Ci::Pipeline.last.iid).not_to be_nil
               end
             end
           end