agent2: Initial infra for checkpoints and message editing (#36120)

Release Notes:

- N/A

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Co-authored-by: Antonio Scandurra <me@as-cii.com>
23cd5b59b2Ben Brandt committed on 8/13/2025, 3:46:28 PM· committed by GitHubparentf4b0332
17 files changedLine totals unavailable