User registration blocked when username already taken (despite Localpart template not being enforced)

[kieranlane]

Describe the bug
When a user logs into MAS with their Identity Provider for the first time, they are presented with a screen to create their MXID. These can be enforced by, for example, a LocalPart template - which prevents a user from changing the value grabbed from the IDP.

If a user whose LocalPart template conflicts with an already registered account, a User Exists error appears.

However, this error also occurs despite no LocalPart template being enforced. MAS receives a suggested localpart from the IDP, it conflicts and the error is displayed.

If this happens to a user, and no LocalPart is being enforced, the user should be presented with the screen to create their MXID and a warning noting that the suggested LocalPart is already taken.

To Reproduce
Steps to reproduce the behavior:

1. Setup Keycloak with two users (i.e. adrian.costello and akira.mcgee)
2. Configure MAS with Keycloak, do not configure a LocalPart template
3. Go to deployed Element Web, get taken to MAS, Sign in as adrian.costello via Keycloak.
4. Change the suggested localpart/username from adrian.costello to akira.mcgee and complete account creation
5. Sign out of adrian.costello in Keycloak (or clear cache / different browser / etc.)
6. Go to deployed Element Web, get taken to MAS, Sign in as akira.mcgee via Keycloak.
7. You will be presented with an error as the suggested localpart akira.mcgee is already taken - despite no enforcement of the localpart, so in theory you should be able to select something different

Expected behavior
When a user is presented with an error due to the IDP providing a value that conflicts with another / existing user, if that value isn't enforced (like via a LocalPart template), then the user should still be presented with the screen to create their MXID, but with an error indicating which fields should be changed due to conflict.

Screenshots