External-source enrollment does not link the existing user

## What

Same Azure-AD/external-source linking failure: enrollment/link does not attach the existing user; #12211 cites #11746 and asks for a manual-link workaround of the same gap.

## Want

- #12211 cites #11746
- Asks for a manual-link workaround of the same gap

## Ideas this came from

- [#11746 Azure AD enrollment not working](https://github.com/goauthentik/authentik/issues/11746) — Describe your question/ I'm trying to link Azure AD with Authentik so that users from Azure AD can log in to Authentik without needing to create a username afte
- [#12211 Way to manually link users with a external provider](https://github.com/goauthentik/authentik/issues/12211) — Yes, it wasn't a problem until all of the users had lost their linking to an Entra account (connected services) I can't tell if it's a Authentik problem or caus

The issue names no files, tests, or entry points. Start by reviewing linked issues #11746 and #12211, then investigate the Azure AD enrollment/linking flow; done means an enrollment attaches to the existing user or the requested manual-link workaround is addressed.

Written by the indexing model from the issue text.