Core: Sweep only the transaction's own snapshots after commit

[raunaqmorarka]

After a successful commit, BaseTransaction reads the manifest list of every snapshot added since the base captured before the retry loop, to avoid deleting a committed manifest during clean-up. That set includes snapshots other writers committed while the transaction was open. Those snapshots cannot reference files this transaction wrote, because deletedFiles only holds manifests and manifest lists produced by the transaction's own updates. Reading them is wasted work.

On S3 each manifest list read is a HEAD and a GET. With N concurrent inserts each commit reads the manifest lists of all snapshots committed since it opened, so the burst costs O(N^2) reads. Measured: 25 to 30 foreign snapshots per commit at about 35 ms each.

This change takes the starting snapshot set from base after the retry loop. applyUpdates refreshes base on every attempt and the successful commit uses that field, so current minus base is exactly the snapshots this transaction created. A manifest deleted in one attempt and reused by a retry is still protected, because the transaction's own committed snapshot references it.

Tests:

• a transaction opened before concurrent commits from another table instance, with an injected retry, reads only its own manifest list after commit
• a manifest merged in the first attempt but kept as is by the retry is not deleted

AI assistance (Claude Code) was used to develop this patch.

[uros-b]

Nice change, thank you @raunaqmorarka! Also, well tested. Please ping committeres who are familiar with the transaction commit path / snapshot infrastructure (e.g. @nastra or @amogh-jahagirdar)