[parquet] Add some tests to cover selective list child padding

[alamb]

Draft as it includes #9848 from @HippoBaro as well

• Follow on from feat(parquet): selective null padding for list child readers  #9848 from @HippoBaro

There were a few uncovered cases I found via code coverage, so this PR adds additional coverage

Changes:

• Adds an ArrowWriter round-trip test for List with an item null, a null parent list, and a long string to exercise selective padding end to end.
• Adds a short definition-level bitmap test that covers the unfiltered full-word append path.

[HippoBaro]

Thank you for the improved coverage 🙇