Fix members#new 500 from nil MembershipPricing amounts

[thibaudgg]

Fixes a recurring production 500 on the public member registration form when the live pricing turbo-frame refreshes.

AppSignal

• Incident: CSA Admin / production exception #347
• Exception: ActionView::Template::Error wrapping undefined method 'round_to_five_cents' for nil
• Action: Members::MembersController#new (GET /new)
• Latest failing tenant: seminterra
• First seen 2024-12-03; still open and last seen 2026-09-15

App frames:

• app/models/membership_pricing.rb #add / #prices / #present?
• app/views/members/members/_pricing.html.erb:3
• app/views/members/members/new.html.erb:95-97

Root cause

The pricing partial builds MembershipPricing from the registration form params (turbo-frame refresh on GET /new).

The latest failing request had:

• waiting_basket_size_id set
• waiting_basket_price_extra set (1.0)
• waiting_activity_participations_demanded_annually set (10)
• waiting_depot_id set
• waiting_delivery_cycle_id blank

#activity_participations_prices maps over resolved delivery cycles and returns [counts.min, counts.max]. When no cycle can be resolved (blank cycle id and a depot without kept cycles, or no depot/cycles at all), that map is empty and the method returns [nil, nil]. #add then calls round_to_five_cents on nil and the pricing frame 500s.

This is not a missing basket/depot fee column. Those prices are non-null. The nil is the computed activity-participation price range when there is no delivery-cycle context.

Fix

• Return [0, 0] for activity (and complement) prices when no delivery cycle can be resolved, instead of [nil, nil].
• Treat a nil amount as 0 before rounding, so #add cannot 500.
• Skip extra-price math when the delivery count is zero, so the same incomplete form state cannot divide by zero.

Valid form data with a resolvable cycle is unchanged (existing pricing tests still pass). An incomplete selection now shows no price rather than a 500.

Tests

test/models/membership_pricing_test.rb covers the production-like params (activity + extra + selected depot + blank cycle, depot with no cycles) and the no-depot/no-cycle case.

Do not merge, deploy, or close the AppSignal incident from this PR.