Fix incorrect Not predicate evaluation in filtering

[Flyangz]

This PR fixes a bug in how Not predicates are evaluated against row group statistics.

The Problem:
Previously, Predicate::Not was handled by simply negating the boolean result of the inner predicate's evaluation (!evaluate(inner)). However, since row group statistics checks are existential (returning true if any row in the group matches), simple negation is incorrect for mixed row groups.

For example:

• IsNull returns true if a row group has any nulls.
• Old logic: Not(IsNull) would return false (skip) for a mixed row group containing both nulls and values.
• Correct logic: Not(IsNull) is equivalent to IsNotNull, which should return true (keep) if the group has any non-null values.

The Fix:
I updated the Not evaluation logic to recursively push the negation down to the leaf predicates:

• Applies De Morgan's laws for And/Or (e.g., Not(A And B) -> Not(A) Or Not(B)).
• Inverts comparison operators (e.g., Not(Eq) -> NotEq, Not(Lt) -> Gte).
• Swaps IsNull <-> IsNotNull.

Tests:

• Added test_evaluate_predicate_not_is_not_null and test_evaluate_predicate_not_or to verify that mixed row groups are correctly preserved.

[Copilot]

Pull request overview

This PR fixes a bug in the evaluation of Not predicates against row group statistics in ORC file reading. The old implementation incorrectly used simple boolean negation, which failed for mixed row groups (containing both nulls and values). The new implementation correctly pushes negation down to leaf predicates using De Morgan's laws.

Changes:

• Fixed Not predicate evaluation to use predicate pushdown instead of boolean negation
• Added ComparisonOp::negate() method to support comparison operator negation
• Added comprehensive tests for all Not predicate transformation cases

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/row_group_filter.rs	Reimplemented Not predicate evaluation to recursively push negation down using De Morgan's laws, comparison operator negation, and IsNull/IsNotNull swapping. Added six new comprehensive tests.
src/predicate.rs	Added ComparisonOp::negate() method to return the negated comparison operator for all six comparison types.

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[WenyXu]

Thanks! @Flyangz