Backport 'Clean up orphan indices in elastic test teardown' (19.0)

What does this MR do and why?

Backports the Elasticsearch test-teardown orphan-index cleanup fix to the 19-0-stable-ee branch.

• !240598 (merged) — Clean up orphan indices in elastic test teardown (712a9e191547)

Why

On 19-0-stable-ee, ee/spec/lib/gitlab/elastic/bulk_indexer_spec.rb (and other :elastic specs) fail consistently during setup with:

RuntimeError: Index or alias under 'gitlab-test-projects' already exists.
# ee/lib/search/elastic/helper.rb: create_index
# ee/spec/support/elastic.rb: create_standalone_indices

The test teardown's delete_indices! listed indices to delete via aliases (client.cat.aliases). An orphan index (an index left without an alias, e.g. by a test that crashed between index creation and alias assignment) is never returned, so it is never deleted. The next :elastic group's create_standalone_indices then raises "already exists" and every example in that group fails.

The fix lists all gitlab-test* indices directly (client.cat.indices) and deletes them, catching orphans. This is a test-only change — no product code, no changelog.

Note

This is an unrelated pre-existing test-infrastructure failure surfacing on the 19-0-stable-ee pipeline; it is not caused by any product change. Fixed on master / 19.2 and absent from 19.0 and 19.1. Backported so the stable pipelines are green.

MR acceptance checklist

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

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.