Filters not working, unable to change site ID

Hi

I have been experimenting with the plugin but only been able to get it to work if hard code the site ID constant.

Could you maybe provide some support / direction on where i may be going wrong.

I have tried both the following:
```
add_filter( 'network-media-library/site_id', function( $site_id ) {
return 18;
} );
```

```
add_filter( 'network-media-library/site_id', array( $this, 'multi_media_network_side_id' ), 999, 1 );

public function multi_media_network_side_id( $site_id ) {
$site_id = (int) get_theme_mod( 'network_media_id' );

if ( false !== $site_id ) {
return $site_id;
}

return 0;
}
```

After some debugging the site id filter is being run but does not seem to be applying it the variable.

Start by reproducing the issue with the two network-media-library/site_id filter examples and trace where the filtered site ID is read. Confirm whether a dynamically returned site ID is applied, and document the failing path or add a regression test if the repository provides one.

Written by the indexing model from the issue text.