Mailbox hand-rolled the same creating-time UUID assignment that
App\Models\Concerns\HasUuid already provides to 17 other models under
R11 (URLs address records by UUID, not integer PK), and skipped the
route-key binding that comes with it. Swap onto the trait, and cover
both the assignment and the route key with a test matching the
convention already used for Host/Order.
Also match MailboxFactory's @extends annotation to how the other
factories in the repo write it (Pint's fully_qualified_strict_types
rule turns a fully-qualified docblock reference into an import plus a
short name anyway, so this is what it would end up as either way).
nexxo