[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.

[JingsongLi]

Requirement fit: SUPPORTED. Implementation: CLEAN in this focused pass. Removing the bounded-VARIANT exclusion lets a row-range read use Parquet OffsetIndex instead of decoding a full row group. The added tests compare selected-page output with the ordinary reader across page versions and null rows, measure fewer bytes read, and exercise the unprofitable and missing-index fallbacks. The production change is one guard removal; I found no changed behavior outside that path in the diff. I reviewed code and tests but did not run PyPaimon locally.