Handle dangling reference error from RefExists

What does this MR do and why?

When a repository has a dangling reference (a ref that exists but whose target object is missing, such as a refs/keep-around/<sha> pointing at a garbage-collected commit), RefExists surfaces an opaque Gitlab::Git::CommandError. This breaks repository mirroring and requires a manual console fix. Gitaly gitlab-org/gitaly!8967 now returns a typed error for this case, but Rails must consume it.

This MR:

• Adds Gitlab::Git::ReferenceUnresolvableError carrying the offending reference name and oid. It subclasses Gitlab::Git::CommandError, so existing callers that rescue CommandError keep working.
• Unwraps the new Gitaly::ReferenceUnresolvableError detail (nested in Gitaly::RefExistsError) into the typed error in WrapsGitalyErrors, so callers get an actionable error instead of a generic CommandError.
• Self-heals in Gitlab::Git::KeepAround#execute: when the keep-around ref is dangling, it logs and deletes the ref, then continues. If the commit is present, the ref is re-created; if it is gone, the SHA is skipped. #kept_around? itself still raises, so only execute deletes refs.
• Rescues the errors delete_refs can raise (Repository::GitError, ReferencesLockedError, InvalidRefFormatError) in execute. These are not CommandError subclasses, so without this a failed delete would escape into inline after_commit callers. A failed delete is tracked and the SHA is returned for retry.

The gitaly gem already includes the new proto from gitaly!8967 (merged), so no gem bump is needed.

References

• Customer RFH: https://gitlab.com/gitlab-com/request-for-help/-/work_items/4994
• Gitaly change: gitaly!8967 (merged)

Verification

Check	Before this MR	With this MR
repository.ref_exists?(ref) on the dangling ref	Raised Gitlab::Git::CommandError	Raised the typed Gitlab::Git::ReferenceUnresolvableError, with the ref name and missing object ID
Gitlab::Git::KeepAround.execute(repository, [sha], source: :verifier)	Returned the SHA in the failed-SHAs array; the dangling ref remained	Returned []; the dangling ref was removed and a valid keep-around ref was created
Repeat the keep-around call with the valid ref	Not tested	Returned []; the ref remained valid

How to set up and validate locally

1. Use a Gitaly version that includes gitaly!8967 (merged).
2. Fabricate a dangling keep-around ref in a test repo: write a commit, point refs/keep-around/<oid> at it, then delete the loose object at objects/<oid[0:2]>/<oid[2:]>.
3. In the Rails console:

   p = Project.find(<id>)
   p.repository.ref_exists?("refs/keep-around/<oid>")
   # => raises Gitlab::Git::ReferenceUnresolvableError
   
   p.repository.keep_around("<oid>", source: 'manual')
   # => [] ; logs 'Deleting dangling keep-around reference' and deletes the ref
   
   p.repository.ref_exists?("refs/keep-around/<oid>")
   # => false

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.