Oauth username is preventing to change avatar

Description:

Using Oauth as described in https://github.com/RocketChat/Rocket.Chat/issues/30520#issuecomment-1743483801, it's possible to create username based on email addresses containing a second @, e.g. @username@example.com.

This works fine overall, but some functions like changing the avatar are blocked because "the username is invalid"

Steps to reproduce:

1 - Create user username@example.com, for example using Oauth configured as described in https://github.com/RocketChat/Rocket.Chat/issues/30520#issuecomment-1743483801
2 - Log in with newly-created account, try to change the avatar
3 - Log in with admin account, try to change the user's avatar

Expected behavior:

• User can change his own avatar
• Admin can change the user's avatar

Actual behavior:

• Rocketchat refuses the change, throws "Invalid username" error, unless the username is change to remove the @

Server Setup Information:

• Version of Rocket.Chat Server: 6.3.7
• Operating System: Debian 11
• Deployment Method: tar
• Number of Running Instances: 1
• DB Replicaset Oplog:
• NodeJS Version: 14.21.3
• MongoDB Version: 5.0.18

Client Setup Information

• Desktop App or Browser Version: Any (server-side error)
• Operating System: Any (server-side error)

Additional context

Relevant logs:

No source file or test is identified in the issue. Reproduce the avatar change with an OAuth-created username containing a second @, then trace the server-side validation that rejects the request as an invalid username. Done means both the user and an administrator can change the avatar without removing the @ from the username.