[ING-49] feat(payment-requests): filter by entity

[mariohd]

Context

Multi-entity customers need to scope the payment requests list to a
specific billing entity. The /customers, /invoices, and /wallets
list endpoints already support a billing_entity_codes filter; this
brings /payment_requests in line.

Description

Adds an optional billing_entity_codes array filter to:

• GET /api/v1/payment_requests
• GET /api/v1/customers/:external_id/payment_requests

Behavior:

• Codes are resolved to IDs against current_organization.all_billing_entities. Unknown codes return 404 billing_entity_not_found, matching the pattern used by /customers, /invoices, and /wallets.
• The query filters through customers.billing_entity_id. PaymentRequest delegates billing_entity to its customer (delegate :billing_entity, to: :customer), so no direct column lookup is needed.
• Backwards compatible: the filter is optional and existing callers see no change.

How to test

Manual:

GET /api/v1/payment_requests?billing_entity_codes[]=EU
GET /api/v1/payment_requests?billing_entity_codes[]=EU&billing_entity_codes[]=US
GET /api/v1/payment_requests?billing_entity_codes[]=BOGUS   # → 404 billing_entity_not_found

Automated:

lago exec api bundle exec rspec \
  spec/queries/payment_requests_query_spec.rb \
  spec/requests/api/v1/payment_requests_controller_spec.rb \
  spec/requests/api/v1/customers/payment_requests_controller_spec.rb