chore: clippy cleanups

[Xuanwo]

This PR moves a few unrelated clippy cleanups out of #6168 so the blob empty-range fix can stay focused on the regression it addresses. The changes here are all mechanical simplifications with no intended behavior change.

[github-actions]

Code Review

Clean, mechanical clippy fixes — all changes are straightforward and behavior-preserving. No issues found.

The is_some() + unwrap() → pattern matching conversions are correct, the Self usage in the enum is idiomatic, get_or_insert_with is the right tool for the lazy-init pattern in tests, and the early-return let-else in filtered_read.rs nicely reduces nesting.

LGTM.

[codecov]

Codecov Report

❌ Patch coverage is 85.33333% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/io/exec/filtered_read.rs	82.92%	2 Missing and 5 partials ⚠️
rust/lance/src/dataset/fragment.rs	60.00%	4 Missing ⚠️

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