Preserve nested Variant extension fields in variant_get

[osipovartem]

Summary

• Handle Variant extension fields inside typed Map, Struct, and List results without interpreting their physical Struct storage as a JSON object.
• Preserve requested binary child storage and reject unsupported nested shredded output explicitly.
• Cover nested Variant arrays/objects, JSON null, physical storage shape, and shredded rejection.

Verification

• cargo +1.95.0 test -p parquet-variant-compute --lib (333 passed)
• cargo +1.95.0 clippy -p parquet-variant-compute --all-targets (only existing arrow-data warnings)
• Independent read-only review: approved, no blocking findings; typed-path benchmark showed no reproducible regression.