Add quota exceeded check to IsSchedulable() and refactor
shouldClearStickySession to delegate to IsSchedulable(), eliminating
duplicated logic and fixing missed overload/rate-limit/expired checks.
Frontend displays quota exceeded status independently via quota fields.
Remove threshold-based waiting in both sticky session and antigravity
pre-check paths. When a model is rate-limited, immediately clear the
sticky session and switch accounts instead of waiting for short durations.