fix: validate list pagination inputs

[ryanduguid]

Summary

• accept only positive integer page numbers across paginated list tools
• retain omitted-page and default-page behavior where supported
• reject invalid requests before they reach the Xero API

Validation

• 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.