Experiment with an Auto CTA on usage notifications

We currently show usage notification banners above the chat input at different thresholds (50, 75, 90, 100) with various calls to action (Add budget, update budget, upgrade).

For paid users, we should experiment with setting these CTA to "Use Auto" to drive Auto usage and measure how it impacts credit drawdown. For full exhaustion, we can still prefer showing "Add Budget".

Experiment design:
- Scope: Pro, Pro+, Max, CB, CE (Free/EDU are already Auto only)
- Treatment: CTA nudge towards Auto
- Control: CTA stay as is

Timeline: TBD, potentially Aug 1 to correspond with the next billing cycle