[ecash-agora] Support full-accept agora partials

Summary:
Attempting to create an agora partial with minAcceptedAtoms the same as offeredAtoms will routinely create an unacceptable offer for "large-ish" offeredAtoms (must be above some truncation threshhold, in practice it tends to happen over ~10_000_000_000 atoms)

For this case, we match the quantities on creation to prevent the best-effort calc from creating an unacceptable offer (minAcceptedAtoms exceeding offeredAtoms).

Arguably this type of offer should instead be a one-shot. But either way, we want to prevent the creation of unacceptable offers.

Test Plan: CI tests

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Subscribers: tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D17776