fix(chain): reject an occupied index in insert_spk

[h55n]

Description

Fixes #2279.

SpkTxOutIndex::insert_spk currently accepts a different script at an existing index. The forward lookup is overwritten while the old reverse lookup remains, so the two maps disagree.

This returns false without changing state when the index is already occupied, matching the existing behavior for an occupied script. The method documentation now covers both cases.

Notes to the reviewers

Added regressions for both lookup directions, unused scripts and previously scanned outputs. A rejected insertion preserves the old output and used status, and the rejected script is not tracked.

Testing on the refreshed master base:

• Rust 1.85: bdk_chain with all features, 99 tests/doctests pass, 1 ignored.
• Changed-crate clippy with all targets and features passes with warnings denied.
• Workspace formatting check passes.
• Full workspace pre-push suite has not been rerun on this base.

Changelog notice

Reject duplicate indices in SpkTxOutIndex::insert_spk without changing existing script or output state.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.87%. Comparing base (0790f04) to head (da0dc65).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2335      +/-   ##
==========================================
+ Coverage   78.84%   78.87%   +0.02%     
==========================================
  Files          31       31              
  Lines        6060     6063       +3     
  Branches      288      289       +1     
==========================================
+ Hits         4778     4782       +4     
+ Misses       1203     1202       -1     
  Partials       79       79              

Flag	Coverage Δ
rust	78.87% <100.00%> (+0.02%)	⬆️

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[evanlinjin]

Thank you for the PR. Looks good. Just some suggestions for doc comments before ACK.

[evanlinjin]

A final request - could you please squash the commits into one? Thanks

[h55n]

Thanks, squashed into one commit and applied the doc suggestion.