fix(datafusion): preserve regexp_match null checks

[lance-gatefixer]

An explicit regexp_match(...) IS NOT NULL filter became IsNotNull(IsNotNull(regexp_match(...))), selecting every row, including NULL inputs. The corresponding IS NULL filter selected no rows. Filter coercion also rewrote regex results inside comparisons.

Restrict coercion to the filter root and AND/OR/NOT operands. Explicit null checks and comparisons now retain DataFusion's nullable list result, while bare regex filters keep their existing behavior. Extend the planner tests with matching, nonmatching, and NULL rows, boolean combinations, and value-context preservation.

Validation:

• Before the fix, eight regression cases failed, including the reported six-row reproduction.
• cargo test --locked --profile ci -p lance-datafusion: 198 unit tests and five doctests passed; three existing doctests were ignored.
• cargo fmt --all passed.
• cargo clippy --locked --profile ci --all --tests --benches -- -D warnings passed.

Fixes #9695

[claude]

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[lance-gatekeeper]

✅ Gate recommendation: approve.

The shared filter-planner fix addresses the incorrect row selection in #9695. Explicit null checks and comparisons retain the regex result’s nullable list semantics, while direct boolean filters retain their existing coercion.