Validate export prefix format on offline transfer configuration

Anish Kumarrequested to merge
gitlab-community/gitlab-org/gitlab:anishisbusy/597501-offline-export-prefix-format into master

What does this MR do and why?

The export_prefix on Import::Offline::Configuration is set by the user when they start an offline transfer import, and it goes straight into object storage keys through Import::Offline::ObjectKeyBuilder. Today it only has a presence check. This MR adds a format validation so the prefix can only hold letters, numbers, dashes, and underscores, which matches the format GitLab generates for exports (for example 2026-03-25_18-17-27_export_4B2u4G61). Values with ., /, spaces, or other special characters are now rejected.

Closes #597501.

Screenshots or screen recordings

Not applicable — backend change.

How to set up and validate locally

  1. Run bundle exec rspec spec/models/import/offline/configuration_spec.rb.

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This contribution was prepared with the help of an AI assistant. I reviewed the changes and take responsibility for them.

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