Address code review feedback and align nexchange with plugin conventions

Require only an apiKey plugin param, hard-coding the fixed base and
currency endpoints and auth header. Use the scoped log for progress
output (latest processed ISO date) and let transaction parsing errors
throw so the run exits without silently dropping volume, saving only the
fully processed progress thanks to oldest-to-newest pagination.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: paullinator