Fix ProgressWALIterator search index extraction

[jt2594838]

Summary

• Extract the real search index from write-plan body entries via SearchNode before falling back to WAL metadata.
• Prevent ProgressWALIterator from treating encoded search-index flags as the actual search index.

Tests

• mvn compile -pl iotdb-core/datanode (timed out after 124s in local environment)

[Caideyipi]

The code change itself looks consistent with the existing WAL decoding path: SearchNode.extractSearchIndex(...) matches how WALInfoEntry#getSearchIndex() and WALNode handle encoded search indexes.

One thing I think we should add before merging is a focused regression test for this exact case. The current ProgressWALIteratorTest#searchableEntry helper writes raw search-index longs, so it would not fail without this fix. Please add a case where the WAL body stores an encoded value, for example SearchNode.encodeSearchIndex(12L, true), and assert that ProgressWALIterator returns search index 12L rather than falling back to metadata or treating the encoded negative value as invalid.