fix: like queries with a prefix should be accelerated by btree and zonemap

[jackye1995]

This allows pruning LIKE "foo%" through any index that allows pruning a range of string prefixes, this includes btree and zonemap.

I ended up adding a LikePrefix instead of StartsWith because it seems like DataFusion converts StartsWith to LIKE

[github-actions]

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[github-actions]

PR Review: LIKE prefix acceleration for btree and zonemap

Overall this is a well-structured change with good test coverage. A few issues worth addressing:

Minor: Escape handling in has_wildcard is approximate

The has_wildcard check (expression.rs ~lines 283-298) only handles 1-2 levels of preceding escape characters, so deeply nested escapes like \\\\% (4 backslashes + %) may be misclassified. The actual prefix extraction loop handles escapes correctly though, so this only affects the early-exit path. Since a false positive in has_wildcard is safe (we just attempt prefix extraction on a literal pattern and it correctly falls through), and false negatives mean we skip optimization for exotic patterns, this is low priority — but worth a comment noting the limitation.

Tests

Good coverage of zonemap pruning, btree range conversion, expression parsing, starts_with conversion, and escape handling. The tests verify correctness of zone inclusion/exclusion with well-chosen boundary cases.

[jackye1995]

looks like I did not handle AND properly, will take another pass tomorrow

[codecov]

Codecov Report

❌ Patch coverage is 92.32053% with 46 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/expression.rs	88.88%	31 Missing ⚠️
rust/lance-index/src/scalar.rs	80.00%	5 Missing and 1 partial ⚠️
rust/lance-index/src/scalar/zonemap.rs	97.34%	5 Missing ⚠️
rust/lance-index/src/scalar/btree.rs	96.07%	3 Missing and 1 partial ⚠️

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