PARQUET: Account for custom PageIndexProvider memory

[zhangxinyao88]

Which issue does this PR close?

• Closes Support reporting heap size of custom PageIndexProvider #11028.

Rationale for this change

ParquetMetaData::memory_size() currently accounts for page-index memory only when the provider is the built-in PageIndex. Custom PageIndexProvider implementations are therefore reported as zero even when they own cached index data.

What changes are included in this PR?

• Add a default PageIndexProvider::heap_size() -> usize method returning zero.
• Have the built-in PageIndex delegate to its existing private HeapSize implementation.
• Account for the concrete provider size, Arc allocation overhead, and provider-reported heap allocation in ParquetMetaData::memory_size().
• Add regression coverage for a custom provider that reports heap usage.

Are these changes tested?

• cargo test -p parquet file::metadata --no-default-features
• cargo check -p parquet --example custom_page_index --no-default-features --features arrow
• rustfmt --check on modified files.

This change was developed with AI assistance. I reviewed the implementation and verified the built-in accounting path, the custom-provider regression test, and source compatibility for the existing custom-provider example.

Are there any user-facing changes?

Yes. Custom PageIndexProvider implementations can opt into ParquetMetaData::memory_size() accounting by overriding heap_size(). Existing implementations remain source-compatible and report zero additional heap usage by default.

[etseidl]

Thanks @zhangxinyao88, but I think a PR for this issue is premature. I think we first need to decide on the approach we want to take before we start implementing solutions.

[zhangxinyao88]

Thanks, that makes sense. I’ll mark this as draft until we’ve agreed on an approach in #11028.