BUG: dask.array.broadcast_shapes: missing implementation

This is present in v2025.12 of the spec, but absent for Dask:

```
>       broadcasted_dims = xp.broadcast_shapes(self_broadcast_dims, x_broadcast_dims)
                           ^^^^^^^^^^^^^^^^^^^
E       AttributeError: module 'scipy._external.array_api_compat.dask.array' has no attribute 'broadcast_shapes'. Did you mean: 'broadcast_arrays'?
```