Prevent non-authorized accounts from accessing API

Investigate if we can prevent access by account on the Gateway, otherwise add a config we set for Sandbox only allowing specific accounts.

Start by reviewing how the Gateway handles account access and how Sandbox-specific configuration is defined. Determine whether access can be restricted at the Gateway; otherwise identify the configuration needed so only specified accounts can use Sandbox, with completion shown by unauthorized accounts being denied.

Written by the indexing model from the issue text.