[ING-48] feat(wallets): filter by billing entity

[mariohd]

Context

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

Description

Adds an optional billing_entity_codes array filter to:

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

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 and /invoices.
• The query matches wallets directly stamped with a billing_entity_id and wallets that inherit it from their customer, preserving Wallet#billing_entity's fallback semantics.
• Backwards compatible: the filter is optional and existing callers see no change.
• Side effect: wallet filters are now compacted before being passed to WalletsQuery, so calling /api/v1/wallets without external_customer_id no longer trips the customer_not_found validation.

How to test

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

Automated:

lago exec api bundle exec rspec \
  spec/queries/wallets_query_spec.rb \
  spec/requests/api/v1/wallets_controller_spec.rb \
  spec/requests/api/v1/customers/wallets_controller_spec.rb