The apiStage this plan serves, derived from its name. The per-API Pulumi
names each plan ${apiStage}-${stack} (e.g. deposit-dev); null for any
plan whose name matches no known stage (legacy / shared plans). Derived from
the name — not the apiStage tag — because AWS GetUsagePlans (the list
call) omits per-item tags.
A usage plan reduced to the fields the resolver matches on.