Rename Org Mailbox

Rename the From-name of one of the org's own BYO mailboxes — the UI face of the gateway's PATCH /v1/mailboxes/{id} (T-778).

Scoped to the caller's own org's dedicated pool by the lookup below (never another org's box, never shared operator inventory) — mirrors list_org_mailboxes's WHERE. The gateway rename is a pure DB-row update (see outreach/routers/mailboxes.py — no EmailEngine re-registration, since outreach/routers/send.py builds the From header PER SEND straight off the gateway's display_name column), so this proxy just re-syncs the single local row from the gateway's response afterwards, same as the connect proxy.