Spec helpers for asserting delivered emails

[mroderick]

This PR was motivated by trying to harvest knowledge from a recent agent session and turn it into something that is actively used, rather than stuffing it into the AGENTS.md file, where it can deteriorate over time.

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Adds a spec helper pair that encodes how delivered emails are asserted in this repo, so the knowledge lives in code next to a canary instead of relying on individual specs re-deriving it.

What each piece does

spec/support/helpers/email_helpers.rb

• delivered_emails_to(member_or_address) — the test queue adapter is delayed_job with delay_jobs = false in test, so deliver_later delivers immediately into ActionMailer::Base.deliveries. This helper is the single place that selects by recipient.
• email_html(mail) — the invitation mailers send multipart/mixed > multipart/alternative > text/html, so
  the root mail.body.decoded is empty and the html part must be read directly. premailer-rails inlines
  CSS on delivery as part of that restructuring.

spec/shared_contexts/email_helpers_spec.rb — a canary in the spirit of the existing 'with forgery
protection enforced' canary. It delivers a real WorkshopInvitationMailer.attending email and exercises
both helpers. If the mailer's part structure or the delivery path changes, the canary fails there instead
of silently weakening every email assertion that uses them.

spec/controllers/workshop_invitation_controller_spec.rb is refactored to consume the helpers: its local html_body definition and inline deliveries.find go away.

Review notes

• Included globally via config.include EmailHelpers (mirrors config.include LoginHelpers in the same file); the method names are email-scoped enough to avoid clashes.
• The canary had to live in spec/shared_contexts/, not next to the helper: rails_helper requires spec/support/** and RSpec then discovers the file again, running each example twice.
• Asserting on the delivered html body is intentionally broad ('been confirmed'); the exact promoted copy is asserted where the waiting-list flag matters.
• Left for follow-up: behaves_like_sending_workshop_emails could consume delivered_emails_to too, but it asserts counts and recipients today, so it was left untouched.