Centralize Aggregate FILTER Some(true) Semantics for Consistency and Correctness Hardening

[kosiew]

Summary

Aggregate FILTER row-validity handling is currently fragmented across helper and grouped accumulator paths. We should centralize the invariant that a row passes aggregate FILTER if and only if the predicate is Some(true).

Current State

• filter_to_validity was introduced and grouped-accumulator helper paths were updated (including accumulate_multiple and filter_to_nulls plumbing) in fix: Incorrect behavior for FILTER on NULLs #22068
• first_last.rs still uses local filter checks in grouped paths.
• accumulate_indices still repeats 64-bit validity iteration logic across multiple branches.

Problem

Duplicated nullable-filter logic increases the risk of semantic drift and regressions (especially around NULL predicate handling), and makes validity bitmap code harder to maintain.

Because grouped and non-grouped paths can evolve independently, this drift can become user-visible as incorrect aggregate results under nullable predicates.

Proposal

1. Add one shared helper/iterator in functions-aggregate-common that encodes:
   • row passes FILTER iff predicate is Some(true)
2. Migrate grouped first_value / last_value filter paths to this shared utility.
3. Refactor accumulate_indices to reuse shared validity iteration and remove duplicated loops.
4. Migrate remaining grouped accumulators incrementally.

Scope

In scope:

• Shared FILTER validity/predicate utility.
• Targeted migration of first_last grouped paths.
• Targeted cleanup in accumulate_indices.
• Focused test coverage for nullable FILTER predicates.

Out of scope:

• Broad performance rewrites not tied to FILTER semantics.
• Large cross-workspace refactors.

Acceptance Criteria

1. A single reusable utility encodes FILTER pass criteria as Some(true).
2. Grouped first_value / last_value no longer use bespoke nullable filter checks.
3. accumulate_indices no longer carries duplicated validity loop logic that can drift semantically.
4. Existing aggregate FILTER tests pass.
5. New tests cover nullable FILTER predicates for grouped first/last behavior.
6. No regression for non-null filter predicates.

Testing

• Add SQLLogicTests for grouped first_value and last_value with nullable FILTER predicates.
• Include cases where value bits may be true while validity is NULL.
• Validate result is NULL when no rows satisfy Some(true).
• Run relevant aggregate crate tests and sqllogictest aggregate coverage.