[python] Read VARIANT pages with Parquet OffsetIndex

[XiaoHongbo-Hope]

What

• Extend the Parquet OffsetIndex row-window reader from [python] Read Parquet row windows with OffsetIndex #9850 to standard VARIANT columns.
• Read only the pages needed by a contiguous row window when it is beneficial.
• Keep the ordinary reader fallback for missing indexes, scattered ranges, and unprofitable selections.

I/O trade-off

Selective reads can transfer fewer bytes but may issue more object-store range requests. This change does not alter the table format or page-index option semantics.