[bhuvan-somisetty]

Description

Fixes an issue where SpkTxOutIndex::is_used returns true for an index that was never tracked or added to the index (e.g. on an empty index or querying an unrevealed keychain derivation index in KeychainTxOutIndex::is_used).

is_used previously checked !self.unused.contains(index). Because self.unused only tracks indices that were explicitly added to the index, any index that has never been tracked is absent from self.unused, causing is_used to evaluate to true.

This PR updates SpkTxOutIndex::is_used to verify self.spks.contains_key(index) && !self.unused.contains(index), matching the guard already present in unmark_used. It also adds documentation and unit tests for both SpkTxOutIndex and KeychainTxOutIndex.

Closes #2444.

Notes to the reviewers

• SpkTxOutIndex::unmark_used already explicitly checks if !self.spks.contains_key(index) { return false; }. Applying self.spks.contains_key(index) to is_used brings full symmetry between both methods and preserves the invariant that untracked indices are not considered used.
• Added unit tests in test_spk_txout_index.rs and test_keychain_txout_index.rs.

Changelog notice

• Fixed SpkTxOutIndex::is_used returning true for untracked script pubkey indices.

Checklists

All Submissions:

• I followed the contribution guidelines

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 83.30%. Comparing base (f2528da) to head (aa79e14).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2445      +/-   ##
==========================================
+ Coverage   83.25%   83.30%   +0.04%     
==========================================
  Files          31       31              
  Lines        6236     6236              
  Branches      293      294       +1     
==========================================
+ Hits         5192     5195       +3     
+ Misses        976      973       -3     
  Partials       68       68              

Flag	Coverage Δ
rust	83.30% <100.00%> (+0.04%)	⬆️

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