Support non-Literal default value for `LAG/LEAD`

### What is the problem the feature request solves?

Comet's native window execution path for `lag` / `lead` only accepts a **literal** as the third (default) argument. Queries like:

```sql
SELECT
b,
LAG(a, 1, c) OVER (ORDER BY b) AS lg,
LEAD(a, 1, c) OVER (ORDER BY b) AS ld
FROM t
```
…where the default expression is a column reference (or any non-Literal expression) currently fall back to Spark. Spark itself supports any expression in this position — the default argument is typed
as Expression, not Literal — so users hitting this pattern lose Comet acceleration on otherwise-supported window plans.

Depends on https://github.com/apache/datafusion/issues/22082

### Describe the potential solution

_No response_

### Additional context

_No response_