feat(snapshot): persist snapshot backfill epoch in fragment stream node

[wenym1]

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What's changed and what's your intention?

Previously in snapshot backfill, we don't persist the epoch on which we read the snapshot, because we didn't have to support recoverable snapshot backfill yet, and we only need to store the epoch in memory, and when recovery happens, either the snapshot backfill is finished, or the job is removed and clean. But to support recoverable snapshot backfill, we may need to persist the snapshot epoch so that we can obtain the snapshot epoch after recovery.

In this PR, we will store the snapshot backfill epoch in the StreamScanNode. In frontend when the node is just created, the epoch field is None. The snapshot backfill epoch is later filled by the global barrier worker when handling the command. When streaming job with snapshot backfill is being created, we will fill the latest upstream epoch to the field before sending it to CN local barrier manager. When the first barrier to create the job is collected and committed, we will update the fragment info in catalog in the post_collect method of Command so that when recovery happened, we can still read the filled snapshot backfill epoch.

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[wenym1]

• feat(snapshot): persist snapshot backfill epoch in fragment stream node #19751 👈 (View in Graphite)
• fix(snapshot-backfill): fix incorrect recv barrier and add test for recover #19756
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[hzxa21]

Rest LGTM