Simplify process_swap_request

[ok300]

Description

This PR slightly changes the process_swap_request implementation: it validates if the tx is balanced before storing the new proofs in the DB. By doing this, we avoid having to later remove these proofs in case the tx is not balanced.

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Notes to the reviewers

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Suggested CHANGELOG Updates

CHANGED

ADDED

REMOVED

FIXED

• NUT-17: Removed superfluous proof state notification

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Checklist

• I followed the code style guidelines
• I ran just final-check before committing

[thesimplekid]

Since we now expect it to fail at the unbalanced check we should change this error

cdk/crates/cdk-integration-tests/tests/fake_wallet.rs

Line 1079 in c620033

cdk::Error::TokenAlreadySpent => (),