Fix events coach/student 500 when member already has an event invitation

[mroderick]

Closes #2794

Summary

Fixes a 500 when a logged-in member clicks Coach/Student on an event where they already have an Invitation row for that event + role with attending NULL.

Related Rollbar items

• Rollbar 712 — EventsController#student, event 448 (virtual-open-day-8)
• Rollbar 713 — EventsController#coach, event 448 (virtual-open-day-8)

Both are the same defect triggered through the student and coach actions respectively.

Root cause

find_invitation_and_redirect_to_event used Invitation.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 token: nil. Because Invitation#to_param returns token, the subsequent redirect_to event_invitation_path raises ActionController::UrlGenerationError. Same bug as #2790, on the events/Invitation path — #2791 only covered workshops.

Fix

After create_or_find_by, check persisted?. If not persisted, fall back to Invitation.find_by(event:, member:, role:) to return the existing row.

Verification

• Added controller specs for GET #student and GET #coach covering:
  • Existing nil-attending invitation for the event + role redirects to the existing invitation page.
  • No new invitation is created in that case.
  • Member without an invitation still creates one and redirects.
• Reproduced the exact production scenario against the codebar_production_dump for both Rollbar items:
  • 712: members 3863/29087 for event 448 (virtual-open-day-8), Student role
  • 713: members 19217/29875 for event 448 (virtual-open-day-8), Coach role
  • All four now route to their existing invitation with a valid /events/virtual-open-day-8/invitation/:token URL.
• Ran controller + request specs: 145 examples, 0 failures.
• bundle exec rubocop app/controllers/events_controller.rb spec/controllers/events_controller_spec.rb — no offenses.

Post-Deploy Monitoring & Validation

• Monitor error tracking for ActionController::UrlGenerationError with controller: "invitations" and token: nil in EventsController#find_invitation_and_redirect_to_event.
• Expected signal: zero occurrences across both Rollbar items (712 and 713) after deploy.