Add snapshot completeness validation

[corylanou]

Summary

• Adds validation to ensure snapshot LTX files contain all expected pages
• Implements the TODO comment in decoder.go about verifying last read page equals commit for snapshots
• Adds comprehensive tests for snapshot completeness validation

Changes

• Modified decoder.go to track the last page number read and validate it matches the expected value for snapshots
• Added TestDecoder_SnapshotCompleteness with four test cases:
  • Complete snapshot validation
  • Incomplete snapshot detection
  • Special handling for lock page boundary
  • Non-snapshot files are not validated

Implementation Details

The decoder now tracks lastPgno during page reads and validates during Close() that:

• For snapshots (MinTXID=1), all pages from 1 to Commit have been read
• Special case: when Commit equals the lock page number, we expect the last page to be Commit-1 (since the lock page is never written)
• Non-snapshot files are not subject to this validation

This ensures snapshot files are complete and haven't been truncated or corrupted.

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