fix: support idempotent Payroll NZ timesheet mutations

[ryanduguid]

Summary

• expose validated idempotency keys for create, add-line, update-line, approve, and revert Payroll NZ timesheet operations
• forward each key to the matching Xero SDK API argument
• keep retry metadata out of the Timesheet request body

Validation

• npm test -- payroll-timesheet-idempotency.handler.test.ts payroll-timesheet-idempotency.test.ts
• npm test
• npm run build
• npm run lint

[ryanduguid]

Closing this one to cut the review load I created here.

I opened a large batch against this repository on 6 August. That was more than any maintainer could reasonably work through, and it was the wrong way to contribute. I have kept open the few changes with the clearest user impact and closed the rest, including this one.

The branch stays on my fork and the change still applies, so if this one is useful, say so and I will reopen it.