[branch-55] fix: correct LEAD/LAG IGNORE NULLS evaluation and limit pushdown (#25472) - #27320

…pache#25472)

## Which issue does this PR close?

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- Closes apache#25471.

## Rationale for this change

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LEAD and negative-offset LAG with IGNORE NULLS can return NULL or a
default value even when the requested non-null row exists.

This happens when stateful evaluation fails to refill a partially
populated lookahead cache, or when window LIMIT pushdown truncates input
before the target row.

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- Resume scanning after the last cached non-null row until enough
candidates are available or the range ends.
- Return `LimitEffect::Unknown` for IGNORE NULLS window UDF expressions,
since non-null offsets cannot bound the number of required input rows.

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Added regression tests in `window.slt` for LEAD/LAG IGNORE NULLS across
NULL gaps and batch boundaries, and in `window_limits.slt` for LIMIT
pushdown correctness.

## Are there any user-facing changes?

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Affected IGNORE NULLS window queries now return the correct results. No
public API changes.

(cherry picked from commit 84acf49)