fix: complex all-null list struct decoding

[Xuanwo]

Fixes #6768.

The v2.1+ complex all-null decoder was slicing repetition/definition buffers with row ranges, which breaks nested layouts where a top-level row expands into multiple list values. This updates the decoder to map requested rows to level slices before materializing the all-null child, and adds coverage for list<struct<...>> where one struct child is all-null across list values.

[github-actions]

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[Xuanwo]

@claude review once

[codecov]

Codecov Report

❌ Patch coverage is 80.28169% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../lance-encoding/src/encodings/logical/primitive.rs	80.28%	11 Missing and 3 partials ⚠️

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[wjones127]

This all looks reasonable. Thanks for the quick fix!