TreeItem.set_custom_as_button doesn't make TreeItem behave as Stylebox

### Tested versions

Reproducible in 4.3.1.rc

### System information

Windows 10

### Issue description

Custom Button Styles are set in Tree but they take no effect.

Result:

### Steps to reproduce

```
extends Tree
@onready var tree: Tree = $"."

func _ready() -> void:
var child := tree.create_item().create_child()
child.set_cell_mode(0, TreeItem.CELL_MODE_CUSTOM)
child.set_custom_as_button(0, true)
child.set_text(0, "This is text")
```

### Minimal reproduction project (MRP)

[custom_as_button_mrp.zip](https://github.com/user-attachments/files/17582231/custom_as_button_mrp.zip)

two files here with Styles set

Start by running the attached custom_as_button_mrp.zip project and reproducing the behavior with the provided Tree and TreeItem GDScript. Trace the TreeItem custom-cell and style handling to determine why set_custom_as_button has no visible effect; done means the configured custom button styles are applied to the TreeItem.

Written by the indexing model from the issue text.