[codex] Use async Artifact Registry image cleanup

[Pigbibi]

Summary

• make the post-deploy Artifact Registry image cleanup asynchronous
• avoid failing deploy workflows when delete operations are issued but operation polling is not available

Validation

• git diff --check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Wait for asynchronous image deletes to finish

In this cleanup loop, adding --async means the workflow exits after each delete operation is submitted rather than after Artifact Registry reports that it completed; the gcloud reference for this flag says it returns immediately without waiting for the operation. When Artifact Registry later rejects a delete (for example a digest that fails a precondition or another backend-side delete error), this step still finishes green, so old Cloud Run images remain in the repository and cleanup regressions are hidden from CI. Please either wait on the returned operations or keep the synchronous delete so failures still fail the workflow.

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