Approve an ISO manager's request to add a new agent.

On success, the person is emailed an invitation to join as an agent on that ISO, and the request's invitationId is set.

The request is checked again at approval. Errors:

• 409 ISO_CAPACITY_EXCEEDED if the ISO already has its maximum number of active agents (100 unless the tenant sets another limit). The request stays pending and blockedReason says why.
• 409 AGENT_BLOCKED_IN_TENANT if the person is blocked in the tenant. Root can still approve.
• 409 ISO_INACTIVE if the ISO has been deactivated.
• 409 USER_EXISTS if the email now belongs to a user.
• 409 INVITATION_EXISTS if the person already has an open invitation.

Available to tenant owners and admins for their own tenant.