[client] Backport SortMergeReader changelog delete fix to 0.9

[luoyuxia]

Purpose

Linked issue: #3134

Backport the fix from #3135 to release-0.9 so SortMergeReader does not terminate early when changelog delete records match snapshot rows during sort-merge reading.

Brief change log

• update SnapshotMergedRowIteratorWrapper#hasNext() to skip empty merge results instead of treating them as end-of-input
• continue advancing until a non-empty merged row is available or the snapshot iterator is actually exhausted
• add the regression test in SortMergeReaderTest to verify later snapshot rows are still returned after changelog deletes filter earlier rows

Tests

• ./mvnw -pl fluss-client -am -Drat.skip=true -DfailIfNoTests=false -Dtest=SortMergeReaderTest test

API and Format

No API or storage format changes.

Documentation

No documentation changes are required.