fix: purge may not work on tables after a flash back operation

[SkyFan2002]

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Summary

On tables that have undergone a FLASHBACK operation, this assumption is broken:

// Take the prev snapshot as base snapshot to avoid get orphan snapshot.

In this PR, the garbage collection root is determined by starting from the current snapshot and traversing backward to the target time point using the prev relation between snapshots. This approach enhances safety (allowing tables modified by the ALTER ... FLASHBACK operation to be handled correctly) but may require traversing a (much) longer history of the table being purged.

Tests

• [ ] Unit Test
• [ ] Logic Test
• [ ] Benchmark Test
• [x] No Test - use existing tests

Type of change

• [x] Bug Fix (non-breaking change which fixes an issue)
• [ ] New Feature (non-breaking change which adds functionality)
• [ ] Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• [ ] Documentation Update
• [ ] Refactoring
• [ ] Performance Improvement
• [ ] Other (please describe):

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This change is 

[dantengsky]

LGTM Maybe need to add some notes on possible impacts to the summary.

PR summary updated, pls let me know if there are any other concerns.