fix: wire Payroll AU v2 client routing

[ryanduguid]

Summary

• expose payroll_au_v2_api from XeroRuby::ApiClient
• configure its Payroll AU v2 base URL explicitly
• route v2 requests to Payroll rather than the Accounting fallback
• deserialize v2 response models through XeroRuby::PayrollAuV2
• cover URL, helper and namespace behaviour with focused specs

Validation

• Ruby 3.2.11 syntax check passed for every file under lib/ and spec/
• git diff --check

The full RSpec run is left to exact-head Linux CI. The portable Windows Ruby install cannot build the lockfile's native gems because MSYS2 is unavailable (byebug, json, racc and bigdecimal stop installation).

The affected runtime files carry OpenAPI Generator headers, while the generator/template repository is private. This is therefore a narrow interim SDK correction; please apply the same routing/configuration mapping to the owning Ruby templates and regenerate so a later SDK release does not overwrite it.