Fix WorkshopsController#rsvp 500 when member already has a nil-attending invitation - #2791

Closes #2790

Summary

Fixes a 500 error when a member clicks Attend as a coach/student on a workshop where they already have a WorkshopInvitation row for that workshop + role with attending NULL.

Root cause

find_or_create_invitation used WorkshopInvitation.create_or_find_by. In Rails 8.1, create_or_find_by calls the non-bang create. When the uniqueness validation fails (row already exists), it returns an unpersisted record with id: nil. The subsequent redirect_to invitation_path(@invitation) then raises ActionController::UrlGenerationError.

Fix

After create_or_find_by, check persisted?. If the record is not persisted, fall back to WorkshopInvitation.find_by(workshop:, member:, role:) to return the existing row.

Verification

• Added spec/controllers/workshops_controller_spec.rb covering:
  • Existing nil-attending invitation for the workshop + role redirects to the existing invitation page.
  • No new invitation is created in that case.
  • Member without an invitation still creates one.
  • Already-attending member is redirected back with the existing message.
• Ran the new spec and all controller/request specs:
  • bundle exec rspec spec/controllers/workshops_controller_spec.rb — 4 examples, 0 failures
  • bundle exec rspec spec/controllers/ spec/requests/ — 136 examples, 0 failures
• bundle exec rubocop app/controllers/workshops_controller.rb spec/controllers/workshops_controller_spec.rb — no offenses

Post-Deploy Monitoring & Validation

• Monitor error tracking for ActionController::UrlGenerationError with controller: "workshop_invitation", id: nil in WorkshopsController#rsvp.
• Expected signal: zero occurrences after deploy.
• If occurrences persist, verify the member/workshop/role combination and check for invitations that cannot be found by find_by (e.g. unique index vs validation mismatch).