Source connection.save() IntegrityError when the user is already linked

## What

on source connection.save() when the user is already linked (CILogon email_link / OAuth source duplicate key).

## Want

- On source connection.save() when the user is already linked (CILogon email_link / OAuth source duplicate key)

## Ideas this came from

- [#20975 `email_link` throws `IntegrityError` when user already has a connection to the same source (CILogon / multi-identity provider broker scenario)](https://github.com/goauthentik/authentik/issues/20975) — When usermatchingmode = emaillink is configured, and a user who already has a UserSourceConnection to a source logs in via the same source with a different iden
- [#22567 IntegrityError (duplicate key) on connection.save() with OAuth Source](https://github.com/goauthentik/authentik/issues/22567) — This is a follow up to the (closed) issue: https://github.com/goauthentik/authentik/issues/16754 I can confirm that we see the same issue (connect.save() is cal

Start by tracing source connection.save() for the CILogon email_link and OAuth source flows, then review issues #20975 and #22567 for the duplicate-key scenarios. Done means a user already linked to the same source can authenticate again without an IntegrityError.

Written by the indexing model from the issue text.