0.2.8 stored self.api_key on the client and _headers() re-read it per
request, so reading client.api_key and reassigning it after construction
both worked. The refactor handed the key to the internal backends and never
set it on PageIndexClient: reads raised AttributeError and reassignment
silently kept requests on the old key. Same shape as the BASE_URL snapshot
fixed in dea211b — apply the same callable-indirection pattern to api_key
on both CloudBackend and LegacyCloudAPI (which also stops CloudBackend
snapshotting its headers dict at construction).
a9bb019d05