[fix](variant) normalize legacy single-part dot-key subcolumn paths on read

[csun5285]

Old versions (cloud-4.1.2 with variant_max_subcolumns_count=0) store a flat JSON key like {"a.b": 1} as a single PathInData part "a.b" in the segment's ColumnPathInfo protobuf. New master compaction builds query paths via PathInData(string) which splits on dots, so the 3-part path does not match the 1-part tree node, find_leaf returns nullptr, and the read plan silently falls back to DEFAULT_FILL. Compaction then rewrites the rowset with empty values, losing the original data.

Normalize the legacy relative path in VariantColumnReader::init() by re-constructing PathInData from its string form. This splits the single-part dot key into multiple parts so the meta tree shape matches what both compaction schema and query path expect.

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[csun5285]

run buildall