Calculate the association maximum separation using the positional coordinates

Association catalogs have a `hats_assn_max_separation` property which defines the maximum separation between any two points in the catalog.

We should calculate this distance directly from the positional coordinates (ra/dec) of the catalogs participating in the crossmatch, instead of using a pre-existing separation column.

_Originally posted by @smcguire-cmu in https://github.com/astronomy-commons/lsdb/pull/811#discussion_r2223865688_:

"Not all crossmatch algorithms are required to give a separation column, and if they do it might be in different units. And it's annoying for the user to have to provide this. I think it will be easier to just compute the max separation from the ra dec columns."

Start by locating the code that sets the hats_assn_max_separation property during catalog crossmatching and inspect how it currently uses a separation column. Trace the ra/dec inputs from the participating catalogs. Done means the maximum separation is computed from positional coordinates without requiring a pre-existing separation column or assuming its units.

Written by the indexing model from the issue text.