feat: add checkout domains from api-specifications#2432

[cursor]

Summary

Adds the public Checkout Domains resource and folds in the pending DX-906 baseline so the path-validation relaxation from api-specifications#2432 is usable in the Node SDK.

get(), delete(), and verifyPaymentMethod() accept plain string IDs and pass them directly into PathParameters, including legacy UUIDs. The additive baseline makes this a minor SDK change, so the package version is bumped to 3.9.0.

Linear issue: DX-1060

Spec-to-SDK mapping

Spec change	SDK files
Relax domain_id for get, delete, and verify-payment-method	src/resources/checkout-domains/index.ts, src/__tests__/resources/checkout-domains.test.ts
Add the pending public Checkout Domains baseline (list/get/delete/verify)	src/resources/checkout-domains/**, src/paddle.ts, src/resources/index.ts
Add baseline response/entity models and enums	src/types/checkout-domains/**, src/entities/checkout-domains/**, src/enums/checkout-domains/**, their root barrels, and src/__tests__/mocks/resources/checkout-domains.mock.ts
Document and version the additive SDK surface	CHANGELOG.md, package.json

Intentionally excluded

• PATCH /checkout-domains/{domain_id} remains @internalUseOnly.
• POST /checkout-domains/{domain_id}/resubmit and POST /checkout-domains remain @partnerOnly.
• No webhook/notification mirrors were added because the spec change has no notification surface.
• The response entity ID remains a normal response string; no path regex, branded type, or runtime validation was introduced.

Validation

• pnpm install --frozen-lockfile — passed
• pnpm build — passed
• pnpm typecheck:tests — passed
• pnpm test --runInBand — 35 suites and 279 tests passed
• pnpm lint --ignore-pattern src/version.ts — passed (the generated, git-ignored src/version.ts has the documented pre-existing eol-last issue)
• pnpm prettier — passed
• Targeted Checkout Domains regression suite — 4 tests passed, covering list plus UUID get/delete/verify URLs

This PR was created by an automated spec-sync agent.