fix(types): avoid reachable panics in Utxo::txout for Foreign variant

[muhahahmad68]

Description

Utxo::txout() contained two reachable panics for the Foreign variant:

1. prev_tx.output[outpoint.vout as usize] — panics if non_witness_utxo has fewer outputs than outpoint.vout
2. unreachable!("Foreign UTXOs will always have one of these set") — asserts an invariant the type does not enforce, reachable via direct construction of Utxo::Foreign with an empty psbt::Input

Addresses part of #474

Notes to the reviewers

Collapsed the two if let arms and the unreachable! into a single chained Option, replacing the index access with .get() to safely handle out-of-bounds vout, and replacing unreachable! with .expect().

This is the short-term non-breaking fix. A follow-up will change the function signature to pub fn txout(&self) -> Option<&TxOut> as a breaking change targeting Wallet 4.0.0.

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

New Features:

• N/A

Bugfixes:

• This pull request breaks the existing API
• 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 80.55%. Comparing base (4b612f5) to head (edb0f52).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #487      +/-   ##
==========================================
+ Coverage   80.30%   80.55%   +0.25%     
==========================================
  Files          24       24              
  Lines        5417     5472      +55     
  Branches      245      243       -2     
==========================================
+ Hits         4350     4408      +58     
+ Misses        989      987       -2     
+ Partials       78       77       -1     

Flag	Coverage Δ
rust	80.55% <100.00%> (+0.25%)	⬆️

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[ValuedMammal]

ACK edb0f52