feat(wasm-utxo): add PSBT comparison utilities for testing

Add low-level PSBT parsing and comparison utilities that work at the
key-value pair level. These utilities provide detailed error messages
showing exactly which fields differ between two PSBTs, making debugging
test failures much easier.

Issue: BTC-2893

Co-authored-by: llm-git <[email protected]>

Author: OttoAllmendinger

packages/wasm-utxo/src/fixed_script_wallet/test_utils/mod.rs

@@ -1,6 +1,7 @@
 //! Test utilities for fixed_script_wallet module
 
 pub mod fixtures;
+pub mod psbt_compare;
 
 use super::wallet_keys::XpubTriple;
 use super::wallet_scripts::{Chain, WalletScripts};