fix(ui): drag and drop not working for sortable hasMany fields (#15845)

### What

Fixes drag-and-drop sorting for relationship and select fields with
`hasMany` and `admin.isSortable: true`.

### Why

In v3.78.0, commit 418bb928ae replaced `PointerSensor` with
`MouseSensor` + `TouchSensor`. The `MultiValue` component was overriding
the `onMouseDown` handler from `listeners`, preventing the `MouseSensor`
from detecting drag start events.

### How

Extract `onMouseDown` from `listeners` and call it explicitly before the
custom `stopPropagation` logic. Touch and keyboard handlers remain
unaffected.

Fixes #15843