Follow-up from "Add developer documentation for Pull mirroring process"

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The following discussion from !172707 should be addressed:

• @jwoodwardgl started a discussion: (+1 comment)

We have added the main flow for pull mirroring. In a follow-up we should add the tangential details so it's easy to see exactly what happens. I think it's a good idea to keep these separate from the main flow, maybe something like

## Stage 2: `ProjectImportScheduleWorker` - Pre-verification

The [`ProjectImportScheduleWorker`](https://gitlab.com/gitlab-org/gitlab/-/blob/f77763345e35c7c4c6a4320f1bd1633a392e3e7c/ee/app/workers/project_import_schedule_worker.rb) performs several checks before scheduling the pull mirroring process:

- Verifies if the project still exists.
- Changes the project’s import status to `scheduled`.
- Marks the project as `failed` if it is no longer eligible for pull mirroring.

The `project.import_state.schedule` call triggers a series of actions, ultimately scheduling the [`RepositoryUpdateMirrorWorker`](https://gitlab.com/gitlab-org/gitlab/-/blob/f77763345e35c7c4c6a4320f1bd1633a392e3e7c/ee/app/workers/repository_update_mirror_worker.rb):

1. [`import_state.schedule`](https://gitlab.com/gitlab-org/gitlab/-/blob/f77763345e35c7c4c6a4320f1bd1633a392e3e7c/ee/app/workers/project_import_schedule_worker.rb#L29)
1. [`ProjectImportState#after_transition`](https://gitlab.com/gitlab-org/gitlab/-/blob/f77763345e35c7c4c6a4320f1bd1633a392e3e7c/app/models/project_import_state.rb#L49-58)
1. [`EE::Project#add_import_job`](https://gitlab.com/gitlab-org/gitlab/-/blob/f77763345e35c7c4c6a4320f1bd1633a392e3e7c/ee/app/models/ee/project.rb#L778)
1. [`RepositoryUpdateMirrorWorker`](https://gitlab.com/gitlab-org/gitlab/-/blob/f77763345e35c7c4c6a4320f1bd1633a392e3e7c/ee/app/workers/repository_update_mirror_worker.rb#L3)

### Additional details

1. [`Gitlab::Metrics#add_event(:mirrors_scheduled)`](https://gitlab.com/gitlab-org/gitlab/-/blob/f77763345e35c7c4c6a4320f1bd1633a392e3e7c/ee/app/models/ee/project.rb#L795)
1. [`Project#log_import_activity`](https://gitlab.com/gitlab-org/gitlab/-/blob/f77763345e35c7c4c6a4320f1bd1633a392e3e7c/app/models/project.rb#L1491-1500)
1. [`ProjectImportState#update` job ID and correlation ID](https://gitlab.com/gitlab-org/gitlab/-/blob/f77763345e35c7c4c6a4320f1bd1633a392e3e7c/app/models/project_import_state.rb#L55)