[combobox] Popup closes automatically when list is filtered in `multiple`

# Bug report

## Current behavior

In the `multiple` case, the popup stays open when items are toggled. However, the popup will close after each item toggle if the list happens to be filtered.

## Expected behavior

I think it's generally better for the popup to stay open regardless of whether the list is filtered.

## Reproducible example

As seen in the [official example](https://base-ui.com/react/components/combobox#multiple-select):

https://github.com/user-attachments/assets/acec37df-010b-48e3-88c4-0c4627105e48

## Base UI version

1.0.0-beta.4

## Which browser are you using?

Chrome

## Which OS are you using?

Mac OS