scaling a pattern to fill a selection in Photoshop

Or will another Photoshop tool do this?

Data-driven graphics can automatically scale to layer bounds, but I am not aware if scripts can utilize this feature. This functionality can certainly be applied manually or by using a pre-existing data list, especially when aiming to populate a defined area with multiple pre-prepared images.

May be useful

https://helpx.adobe.com/photoshop/using/creating-data-driven-graphics.html

https://helpx.adobe.com/photoshop/using/create-smart-objects.html

One thing you could do is clip the circle layer to the image layer and then resize the image layer to cover (in either order). Scaling to cover means you calculate the scale factor to match both height and width and then take the maximum. When doing this, I typically scale it an extra pixel on each side to prevent anti-aliased edges from showing up in the clipped area. Below is the scriptListener function that clips the active layer to the one below.

function ClipActiveLayer() {
  var d = new ActionDescriptor()
  var r = new ActionReference()
  r.putEnumerated( chID( "Lyr " ), chID( "Ordn" ), chID( "Trgt" ) )
  d.putReference( chID( "null" ), r)
  executeAction( chID( "GrpL" ), d, DialogModes.NO )
}

You may need to write a custom script. You can get the size by selection.bounds and then get the fill bounds (say, another layer) using layer.bounds then scale with layer.resize().