Deduplicate the registrant CSV row builder (events vs event_registrations exports)

🤖 From Claude: follow-up noted during the events-IA refactor (PR #2113); out of scope there.

Summary

Two controllers build essentially the same per-registrant CSV row from scratch:

• EventsController#event_registration_csv_row (app/controllers/events_controller.rb, ~L876) — the per-event Manage roster export.
• Events::EventRegistrationsController#csv_export (app/controllers/event_registrations_controller.rb, ~L365) — the cross-event registrations export.

Both emit the same columns — first name, last name, email, phone, org(s), scholarship (yes/no), scholarship completed, payment status, intends to pay, payment total, and CE status/paid/due — with only minor differences:

• The cross-event export adds an Event column and uses attendance_status_label; the per-event one omits Event and gates payment on cost_required.
• Org names: the per-event builder derives active-affiliation org names in Ruby; the cross-event one omits orgs.
• CE columns are always present cross-event; conditional (include_ce) per-event.

They share csv_dollars, allocations_sum, scholarships.any?, payment_status_label, etc. — so the cell logic is duplicated and can drift (a change to how, say, scholarship-completed or CE-due is computed must be made in both).

Proposed approach

Extract a single row/column builder (e.g. an EventRegistrationCsvRow PORO or a shared concern) that takes an EventRegistration plus options (include_event_column:, include_ce:, cost_required:) and returns the header + row. Both controllers call it. Keep the two call sites' column-set differences as explicit options rather than divergent implementations.

Tasks

• Extract the shared header + row builder.
• Point both controllers at it (preserving each export's current columns exactly).
• Spec the builder directly; keep the existing controller CSV request specs green (byte-for-byte output unchanged).

Out of scope

• No new columns or format changes — pure dedup.

References

• PR #2113 — the events-IA refactor that surfaced this.

Start with EventsController#event_registration_csv_row in app/controllers/events_controller.rb and Events::EventRegistrationsController#csv_export in app/controllers/event_registrations_controller.rb. Compare their headers and row logic, then review the existing controller CSV request specs before choosing the shared builder shape. Done means both controllers use the builder, a direct builder spec covers its options, and existing CSV output remains byte-for-byte unchanged.