fix: expose Payroll AU v2 API client

[ryanduguid]

What changed

• export the already-generated PayrollAuV2Api
• construct it on XeroClient as payrollAUV2Api
• propagate access-token changes through one client registry
• cover construction and token propagation with regression tests

Why

The generated Payroll AU v2 class exists, but it is omitted from the public API barrel and XeroClient. Consumers therefore cannot use it through the supported client surface, and token refreshes do not reach it.

Generator follow-up

src/gen/api/apis.ts is generated code. This patch is a narrow interim SDK correction; the owning generator/template should add Payroll AU v2 to its API registry so the fix survives regeneration.

Validation

• npm ci
• npm run build
• npm test -- --runInBand — 17 tests passed
• git diff --check