Account products no longer auto-create agent lines

New accounts no longer receive a default agent line product at signup. Add messaging products through account products when your integration is ready, instead of inheriting a line you may not use.

• GET /v3/accounts/{account-id}/products on a new account lists only products you created; nothing is provisioned automatically when the account is created.
• Existing accounts keep agent line products already on the account; only new account onboarding changes.