[ecash-agora] Do not validate for unspendable offers when estimating the fee

Summary:
Open question whether or not we should support allowing the creation of unacceptable offers in this lib at all. Unrelated to this diff tho. For now, this is something that can be manually specified in the lib, because it is something the protocol technically allows.

However, when we just want to estimate a fee, we should not be validating for this condition. Just estimate the fee.

Test Plan: CI tests

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D17648