* feat(auth): discontinue Qwen OAuth free tier (2026-04-15 cutoff)
The Qwen OAuth free tier has reached its end-of-life date. This updates
all client-side messaging, blocks new OAuth signups, and guides existing
users to alternative providers.
* fix(test): add getModelsConfig mock and update QWEN_OAUTH test expectations
- Add getModelsConfig() to Config mocks in gemini.test.tsx (3 failures)
- Update validateNonInterActiveAuth test to expect exit for QWEN_OAUTH
since validateAuthMethod now returns an error for discontinued free tier
