Test adapter for Accrue.Mailer (D6-05). Replaces Accrue.Mailer.Default
at the behaviour layer — intercepts Accrue.Mailer.deliver/2 BEFORE
Oban enqueue. Sends the intent tuple to the calling test pid and
returns immediately.
Wired via config :accrue, :mailer, Accrue.Mailer.Test in test env.
Mirrors Accrue.PDF.Test (Phase 1 D-34) — every Accrue test adapter
sends a 3-tuple starting with a :*_rendered/:*_delivered atom.
For tests that need a rendered %Swoosh.Email{} body (subject / HTML
assertions), swap this adapter for Accrue.Mailer.Default plus
Swoosh.Adapters.Test in that specific test module.