Allow elastic client adapter to be set

What does this MR do and why?

This MR re-instates a reverted MR. I added the fix in a separate fixup commit to ease review from the original MR

The fix included:

• add null check operator when using options[:client_adapter]
• adding .to_sym to value
• adding some tests which fail before the fix

References

merge requests

• Original: !204437 (merged)
• Revert: !205662 (merged)

issues

• TCP connections from GitLab processes accumulat... (#550805 - closed)
• Reinstate https://gitlab.com/gitlab-org/gitlab/... (#571222 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. enable elasticsearch for gdk
2. setup a new connection for the elastic adapter

   es_connection = Ai::ActiveContext::Connection.create!(name: 'es-test-adapter', adapter_class: "ActiveContext::Databases::Elasticsearch::Adapter", options: { use_advanced_search_config: true } )

3. activate connection

   es_connection.activate!

4. reload or restart rails console

   reload!

5. run the migration worker

   Ai::ActiveContext::MigrationWorker.new.perform

logs before

{
  "severity": "INFO",
  "time": "2025-09-19T12:59:27.664Z",
  "class": "Ai::ActiveContext::MigrationWorker",
  "message": "Ai::ActiveContext::MigrationWorker: Created missing migration records for 20250423135734, 20250430114936, 20250508151423",
  "job_status": "running",
  "queue": "default",
  "jid": null
}
{
  "severity": "INFO",
  "time": "2025-09-19T12:59:27.667Z",
  "class": "Ai::ActiveContext::MigrationWorker",
  "message": "Ai::ActiveContext::MigrationWorker: Starting migration 20250423135734",
  "job_status": "running",
  "queue": "default",
  "jid": null
}
{
  "severity": "INFO",
  "time": "2025-09-19T12:59:27.672Z",
  "class": "Ai::ActiveContext::MigrationWorker",
  "message": "Ai::ActiveContext::MigrationWorker: Migration 20250423135734 failed: wrong constant name typhoeus. Retries left: 2",
  "job_status": "running",
  "queue": "default",
  "jid": null
}

logs after

{
  "severity": "INFO",
  "time": "2025-09-19T13:00:31.529Z",
  "class": "Ai::ActiveContext::MigrationWorker",
  "message": "Ai::ActiveContext::MigrationWorker: Starting migration 20250423135734",
  "job_status": "running",
  "queue": "default",
  "jid": null
}
{
  "severity": "INFO",
  "time": "2025-09-19T13:00:31.554Z",
  "class": "Ai::ActiveContext::MigrationWorker",
  "message": "Ai::ActiveContext::MigrationWorker: Marking migration 20250423135734 as completed",
  "job_status": "running",
  "queue": "default",
  "jid": null
}

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Related to #550805 (closed)