[Index state tracking: Deletion] Trigger ad-hoc indexing for deleted repo

If a repo was marked as deleted due to being stale (not queried in the last 3 months), we should allow ad-hoc indexing to be triggered if it does get triggered again.

JTBD:

• Trigger ad-hoc indexing for repositories that are in deleted state if it gets queried.
• AdHocIndexingWorker needs to be updated since it returns if repo exists
• ee/lib/ai/active_context/queries/code.rb needs to be updated to call try_trigger_ad_hoc_indexing if the repo exists but is in deleted state.