Internal HTTP layer shared by every toa_*() wrapper. Builds an httr2
request from a base URL plus a named list of query parameters, applies the
standard user agent + transient-error retry policy, performs it, and caches
the x-requests-* usage-quota headers for toa_quota(). NULL query values
are dropped automatically by httr2::req_url_query(), so optional parameters
can be threaded through unconditionally.