Assert the waiting-list promotion email on reject

[mroderick]

Every waiting-list promotion emails the promoted member via WorkshopPresenter#send_attending_email(invitation, true), and no spec asserted any of it — only the resulting invitation state. This PR pins the email for the reject flow.

One of five small PRs from the same pre-existing-gap audit. Heads are reviewed independently; if a sibling merges first expect a trivial rebase.

Detail

Three assertions, inside the existing "when someone are on waiting list" reject context:

• The promoted member receives an attending confirmation (matched by recipient).
• The body uses the promotion copy — only rendered when waiting_list is true; the plain accept copy says something else.
• Exactly one email goes out in total, so the rejecting member gets no promotion copy.

Repo specifics worth knowing for review. The test queue adapter is delayed_job; delay_jobs = false in
test means deliver_later delivers into ActionMailer::Base.deliveries. The invitation mailer is
multipart/mixed > multipart/alternative > text/html, so the root body.decoded is empty — the specs
read the nested html part via a small html_body helper.

Review notes

• The copy-match assertion on 'been confirmed' is broad on purpose (view strings change); the flag assertion is the exact promoted copy.
• Deliberately not added here: the admin-removal call site has no equivalent assertion on master — add it with feat/post-close-rsvp-waitlist, which introduces that call site.

Sibling PRs:

• Rejecting drops a cancelling member's waiting-list entry #2997 (fix/waiting-list-self-promotion-on-reject)
• Redirect instead of a 500 when the waiting-list entry is already gone #2998 (fix/waiting-list-destroy-missing-entry)
• Skip CSRF for reject (token-authenticated, no session) #2999 (fix/reject-token-no-session)
• Pin cross-role waiting list promotion #3000 (test/waitlist-cross-role-promotion)