Adds 17 missing exports to the __testing object in usage.ts so
usage-utils.test.ts can import and test them. Also adds the
test file itself.
- toDisplayLabel, getClaudePlanLabel, createQuotaFromUsage
- getMiniMaxQuotaResetAt, isMiniMaxTextQuotaModel
- getMiniMaxSessionTotal, getMiniMaxWeeklyTotal
- createMiniMaxQuotaFromCount, getMiniMaxAuthErrorMessage
- getMiniMaxErrorSummary
- mapCodeAssistSubscriptionToPlanLabel
- mapCodeAssistTierIdToLabel
- mapSubscriptionTierStringToPlanLabel
