Look up multiple products by retailer and product ID in a single
request. Partial success is expected: an unknown retailer, an authorized
retailer, or a product that is not found produces a per-item entry in
errors rather than failing the whole batch. Each item in products
and errors echoes its retailer and product_id so callers can
correlate results back to their request.

Batch items are always looked up in the us region; there is
currently no per-item region override (see region on the
single-product endpoints).