"end to end" test for reading parquet using custom PageIndexProvider

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

#10842 introduces the PageIndexProvider trait so that applications can plug in custom page index implementations (for example, sparse or lazily-loaded indexes) into ParquetMetaData.

However, no test currently drives reading with a custom PageIndexProvider through the actual read path.

Several places in the code use as_any().downcast_ref::<PageIndex>() and take a different path for custom providers (for example ParquetMetaData::memory_size, PartialEq for ParquetMetaData, and ParquetMetaDataWriter), so adding specific reader coverage would be good.

• The custom_page_index.rs example implements OnDemandPageIndexProvider, but it only prints index availability, and examples are compiled — not executed — in CI.
• The existing reader tests exercise the built-in PageIndex implementation only.

Describe the solution you'd like

Add an integration test that:

1. Writes a parquet file with page indexes (multiple row groups, multiple columns, multiple pages per column).
2. Loads the metadata with PageIndexPolicy::Skip, then installs a custom PageIndexProvider implementation (similar to the example's OnDemandPageIndexProvider) via ParquetMetaDataBuilder::set_page_index, populated for only a subset of row groups / columns.
3. Reads data through ParquetRecordBatchReader (and/or the async reader / push decoder) with a RowSelection that triggers page skipping, and verifies:
   • correct results are returned,
   • the offset-index-driven fetch path is used for columns where the provider returns Some,
   • columns where the provider returns None fall back to whole-column-chunk fetching.

Describe alternatives you've considered

Converting the custom_page_index.rs example into an executed test, or adding assertions to it and running it in CI.

Additional context

• Follow-up from review of Add PageIndexBuilder and PageIndexProvider for Parquet page indexes #10842