EDU/workspace accounts report spend_control.individual_limit as null in
wham/usage; the administrator-defined monthly limit only exists on the
separate accounts/{id}/spend-controls/current-user/monthly-usage
endpoint. Fetch it (OAuth and OpenAI web API-first paths) when a
monthly limit is plausible but missing, map it into the existing
CodexCreditLimitSnapshot model, and fail soft on any endpoint error.
Fixes#2900
Peter Steinberger
105a808a4f