[VL] Handle multi-children Count in window expressions

### Backend

VL (Velox)

### Bug description

Velox only supports count() / count(T) for window functions. Spark's
`count(c1, c2, ...)` variant must be rewritten into `count(if(or(isnull(c1),isnull(c2), ...), null, 1))` so the `WindowExec` can still be offloaded, which has already be handled in `AggregateExec`

### Gluten version

main branch

### Spark version

None

### Spark configurations

_No response_

### System information

_No response_

### Relevant logs

```bash

```

Start by locating the WindowExec handling for count expressions and compare it with the existing AggregateExec handling described in the issue. Verify the multi-child count rewrite for null inputs and run the relevant window-function tests; done means Spark-style count(c1, c2, ...) can be offloaded to Velox.

Written by the indexing model from the issue text.