fix(key-link): validate page size before requests

[arunimshukla]

Summary

• parse Key Link page sizes as integers
• enforce the API-supported 1-50 range for signing and validation key listings
• add end-to-end coverage for the upper boundary and client-side rejection

Why

The CLI currently accepts any string for --page-size, so unsupported values are sent to the API and fail with a generic 400 response. Validating the documented range locally gives users an immediate, actionable error and keeps the two Key Link list commands consistent.

Testing

• npm test -- --runInBand (86 tests passing)
• npm run build

Fixes #54