fix: handle non-ascii username collisions

[asajjad2]

What are the relevant tickets?

#8527

Description (What does it do?)

When OpenEdX returns empty username suggestions for usernames containing non-ASCII characters (like "Nguyễn"), the system was raising an OpenEdxUserCreateError instead of falling back to local username generation. This fix implements a fallback mechanism that generates unique usernames locally using random number suffixes when OpenEdX suggestions are unavailable.

Changes

• Added _generate_unique_username() helper function to modularize username generation logic
• Updated _create_edx_user_request() to fall back to local username generation when OpenEdX returns empty suggestions
• Refactored reconcile_edx_username() to use the same modular username generation function

How can this be tested?

1. Create a user with non-ASCII username: Nguyễn. Verify the user gets created both in MITxOnline and edX.
2. Try to create another user with the same username
3. Verify the behavior:
   • The second user should be created successfully (not fail with an error)
   • The second user should get a unique username with a random suffix
   • No OpenEdxUserCreateError should be raised
   • Both users should exist in the system with different usernames