[core] Fix LookupMergeFunction to use sequence.field for picking high level records

[Liulietong]

Purpose

Fix #7220

When sequence.field is configured, LookupMergeFunction.pickHighLevel() should select the record with the highest sequence value instead of the lowest level number.

Problem

Previously, pickHighLevel() only compared level numbers:

if (highLevel == null || kv.level() < highLevel.level()) {
    highLevel = kv;
}

This could lead to incorrect results when out-of-order data arrives:

• L1 has sequence=7 (older)
• L2 has sequence=8 (newer)
• L0 has sequence=6 (oldest, out-of-order arrival)

The old logic would pick L1 (level 1 < level 2), but the correct behavior should pick L2 (sequence 8 > 7).

Changes

1. Add sequenceComparator field to LookupMergeFunction
2. Modify pickHighLevel() to use sequence comparator when available
3. Modify getResult() to sort records by sequence before adding to merge function
4. Only set sequenceComparator when user-defined sequence field is configured, preserving original behavior when sequence.field is not set

Tests

Added 3 test cases:

• testSequenceFieldWithMultipleLevels - verifies sequence.field is used correctly
• testWithoutSequenceFieldPreservesOriginalBehavior - verifies backward compatibility
• testSequenceFieldWithDescendingSortOrder - verifies descending sort order works

Impact

Only affects changelog-producer = lookup with sequence.field configured. No impact on:

• Normal queries (Batch/Streaming Scan)
• Lookup Join
• Tables without sequence.field