fix: handle numbers-only username generation fails

[asajjad2]

What are the relevant tickets?

Fixes https://github.com/mitodl/hq/issues/9423

Description (What does it do?)

When users register with numbers-only names (e.g., 15418306) and their email local part is also numbers-only (e.g., [email protected]), the usernameify function fails with a ValueError. This is because The USERNAME_INVALID_CHAR_PATTERN constant includes [\d] which removes all digits from usernames to comply with Open edX's requirement that usernames cannot be entirely numeric.

This PR, instead of raising a ValueError when both name and email fail to produce a username, we now generate a random username as a fallback. This ensures users can still register successfully even with edge case inputs.

How can this be tested?

1. Register a new user on MITxOnline with a completely numeric name and numeric email.
2. Verify you don't get https://mit-office-of-digital-learning.sentry.io/issues/6903551870/?referrer=github_integration
3. Verify the edx user against your user has a random edx username attached to it

Additional Context

This is mostly a fallback strategy and the original issue would be resolved through validation checks within MITxOnline forms.

[jkachel]

LGTM 👍