[ING-70] feat(multi-entity): billing_entity scoping across endpoints

[mariohd]

Summary

Reflects the multi-entity billing API change in the spec. Callers can now pin a subscription, wallet, or one-off invoice to a specific billing entity within a multi-entity organization, read the resolved entity back from subscription and wallet responses (invoice and credit_note already exposed it), and filter the subscription, wallet, and payment_request list endpoints by billing_entity_codes[] (the invoice and credit_note lists already supported it).

Changes

Schemas — billing_entity_code (string) added to:

• SubscriptionUpdateInput — PUT /subscriptions/:external_id. Mutable, takes effect on future invoices.
• SubscriptionObject — read back on every subscription response.
• WalletCreateInput — POST /wallets (also covers POST /customers/:external_id/wallets, which shares the schema). Defaults to the customer's billing entity.
• WalletUpdateInput — PUT /wallets/:id. The API permits it on update too.
• WalletObject — read back on every wallet response.
• InvoiceOneOffCreateInput — POST /invoices (one-off).

The SubscriptionCreateInput, InvoicePreviewInput, InvoiceBaseObject, and CreditNoteObject schemas already declared billing_entity_code before this PR.

Resources — billing_entity_codes[] array query filter added to:

• GET /subscriptions and GET /customers/:external_customer_id/subscriptions.
• GET /wallets and GET /customers/:external_customer_id/wallets.
• GET /payment_requests and GET /customers/:external_customer_id/payment_requests.

GET /invoices and GET /credit_notes already exposed the filter.

Notes

No new schemas, parameters, or index entries — every edited file was already registered. The regenerated top-level openapi.yaml propagates the changes.

SubscriptionObject.billing_entity_code is added per the target API behaviour; the SubscriptionSerializer on the API side is part of the same in-flight work and should land together with — or just before — this PR.

Verification

• npm run build — redocly bundle clean.
• npm run test — redocly + spectral lint clean (0 errors). The 6 spectral warnings reported are pre-existing (status[], coupon_code[], account_type[], one unused component) — none introduced here. The new billing_entity_codes[] filter is plural and bracket-suffixed so it satisfies both array-params-plural and array-params-brackets.