Ensure after_run ops execute once only

What does this MR do and why?

This change ensures any operations designed to execute after a bulk_import pipeline has run will only be executed once, whether the pipeline is batched or not batched. It corrects the error where after_run operations were run after each batch completion, rather than after all batches were completed. The method/pipeline step has been named 'on_finish' for clarity.

Changelog: fixed

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Enable the :bulk_imports_batched_import_export feature flag.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/429856