Environment-variable names do not prove provider mode

An environment-variable name is not evidence of the provider mode a client library uses. Trace how its value is passed into the SDK and which mode-selecting options, project identifiers, locations, and hosts are actually configured.

Presence validation can prove only that a value exists. It cannot detect service enablement, key restriction, or wrong-provider faults, especially when downstream code translates provider authorization errors into generic application failures.