fix(coin_selection): calculate_cs_result returns the required UTXOs first

[ValuedMammal]

Description

Follow-up to #262 that addresses transaction input ordering when BnB finds a solution.

Previously calculate_cs_result produced a CoinSelectionResult by appending the required UTXOs onto the selected ones, which changed the expected order of transaction inputs.

calculate_cs_result now returns the required UTXOs before the newly selected ones. This behavior aligns with the expectation that the order of manually selected inputs should be preserved in the final transaction whenever TxOrdering::Untouched is specified.

For related discussion refer to #244 (comment).

Changelog notice

Fixed

• wallet: Fixed order of selected UTXOs for BranchAndBoundCoinSelection, required UTXOs come first

Checklists

All Submissions:

• I followed the contribution guidelines

Bugfixes:

• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.77%. Comparing base (c422104) to head (96c62d5).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #390      +/-   ##
==========================================
+ Coverage   79.17%   79.77%   +0.60%     
==========================================
  Files          24       24              
  Lines        5311     5267      -44     
  Branches      242      241       -1     
==========================================
- Hits         4205     4202       -3     
+ Misses       1029      988      -41     
  Partials       77       77              

Flag	Coverage Δ
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[Serious-Sam-Dev]

all test and clippy passed, i dont see any problem, ACK

[110CodingP]

ACK 378cb24

Though unrelated to the PR I was wondering whether we should also have a test to check the following invariant in add_utxos ?
"If a UTXO is inserted multiple times, only the final insertion will take effect."

[ValuedMammal]

Thank you @110CodingP

"If a UTXO is inserted multiple times, only the final insertion will take effect."

We do have these tests, or are you referring to something else?

• duplicated_utxos_in_add_utxos_are_only_added_once
• not_duplicated_utxos_in_required_list

[110CodingP]

Sorry for being brief, I meant something like this:

// This demonstrates that `add_utxo` only considers the final insertion.
#[test]
fn test_add_utxos_final_op_retained() {
    // Create empty wallet
    let (desc, change_desc) = get_test_wpkh_and_change_desc();
    let mut wallet = Wallet::create(desc, change_desc)
        .network(bdk_wallet::bitcoin::Network::Regtest)
        .create_wallet_no_persist()
        .unwrap();

    let outpoint_0 = receive_output(
        &mut wallet,
        Amount::from_sat(35_000),
        ReceiveTo::Mempool(50),
    );
    let outpoint_1 = receive_output(
        &mut wallet,
        Amount::from_sat(25_200),
        ReceiveTo::Mempool(100),
    );

    let send_to = wallet.next_unused_address(KeychainKind::External).address;
    let mut tx_builder = wallet.build_tx();
    tx_builder
        .add_utxo(outpoint_0)
        .unwrap()
        .add_utxo(outpoint_1)
        .unwrap()
        .add_utxo(outpoint_0)
        .unwrap()
        .add_recipient(send_to.script_pubkey(), Amount::from_sat(60_000))
        .fee_rate(FeeRate::from_sat_per_vb(1).unwrap())
        .ordering(bdk_wallet::TxOrdering::Untouched);
    let psbt = tx_builder.finish().unwrap();

    // Fails
    assert_eq!(
        psbt.unsigned_tx
            .input
            .iter()
            .map(|txin| txin.previous_output)
            .collect::<Vec<_>>(),
        vec![outpoint_0, outpoint_1]
    );
}