The interaction contract

Implement MailerSpy and send_welcome(mailer, user): - MailerSpy: send(to, subject, body) records the call and returns True; if fail_next is set, it raises ConnectionError once (and clears the flag) instead - send_welcome: sends ('Welcome!', 'Hi <name>') to user['email']; returns True on success, False when the mailer raised ConnectionError — and on failure it must NOT retry inside the call The spy makes the contract observable: exactly one send on success, exactly zero on failure.

Difficulty: advanced