Page index behavior: handle partial / missing indexes without panics

Goal

Ensure Parquet page indexes behave correctly when some or all columns lack indexes by:

• Handling None at the leaf of ParquetColumnIndex and ParquetOffsetIndex as “no index for this column chunk”
• Avoiding panics in normal usage when indexes are legitimately missing
• Adding tests that lock in the expected behavior for partial and fully missing page indexes

Which issue does this PR close?

Closes #8818.

Rationale for this change

With the earlier PRs:

• Page index types now model per-column optionality as Vec<Vec<Option<...>>>
• Column index semantics are normalized so missing indexes use None

However, several call sites and higher-level APIs still effectively assumed that page indexes were present for all requested columns once page index reading was enabled. This could lead to:

• unwrap() / as_ref().unwrap() panics when a specific column chunk had no index
• Confusing behavior when some columns had page indexes and others did not
• No explicit tests covering these partial- and no-index cases

This PR focuses on making the runtime behavior robust for these scenarios and documenting it via tests.

What changes are included in this PR?

Safer handling of per-column Option page indexes

• Update page-iterator, selection, and in-memory row-group code paths to:

  • Treat None in ParquetOffsetIndex[row_group][column] or ParquetColumnIndex[row_group][column] as “no page index for this column chunk”
  • Fall back to non-indexed behavior (e.g., scanning full columns or skipping page-level pruning) instead of panicking

• Where expect(...) is still used, it is reserved for true internal invariants such as:

  • “page index should be present when page index reading is explicitly enabled and this code path is only constructed under that assumption”
  • These invariants are now spelled out in error messages to make assumptions explicit.

Row selection and read-plan behavior

• RowSelection and related selection helpers are updated to:
  • Take &[Option<OffsetIndexMetaData>] where appropriate
  • Skip page-level pruning for columns whose offset index is None
  • Respect the current RowSelectionPolicy while avoiding panics for missing indexes

In-memory row group and async/arrow readers

• InMemoryRowGroup now:

  • Uses Option<&[Option<OffsetIndexMetaData>]> for offset indexes
  • Uses page indexes where present to compute sparse read ranges
  • Falls back to reading the entire column when no offset index is present, instead of unconditionally indexing into offset_index[idx]

• Async and synchronous readers are updated to:

  • Assume offset/column indexes may be missing on a per-column basis
  • Still assert the invariants they depend on when page index usage is explicitly requested (e.g., in tests that are specifically about index behavior)

PageIndexPolicy behavior

• With per-column Option modeling, parse_offset_index now:

  • Builds a Vec<Vec<Option<OffsetIndexMetaData>>> so missing offset indexes are represented as None
  • Avoids invalidating all page indexes when some columns are missing indexes under non-Required policies

• The practical effect is:

  • For optional page index use, partial presence no longer forces “invalidate everything”
  • Call sites must and do handle per-column None in a defined, non-panicking way

Tests for partial and missing page indexes

• Add focused tests (e.g., in parquet/tests/arrow_reader/statistics.rs) that cover:
  1. All columns have page indexes (baseline behavior)
  2. Some columns have column indexes while others do not:
     • Verify that metadata reading and page-index-based logic do not panic
     • Verify that the “missing” columns are represented as None at the page index leaf
  3. No page indexes at all (page indexes disabled):
     • Verify that metadata reports None for top-level page indexes
     • Verify that reading still works without panics and falls back to non-indexed behavior
• These tests document expected behavior and guard against future regressions.

Are these changes tested?

Yes.

• All existing parquet tests continue to pass.
• New and updated tests are included to explicitly cover:
  • Partial column index presence