feat: expose detailed try_finalize_psbt outcomes

[reez]

Description

First step for #73, following ValuedMammal's suggestion.

This PR keeps the scope to PSBT finalization only:

• add try_finalize_psbt which returns FinalizePsbtResult
• report per input outcomes via FinalizePsbtInputResult

Notes to the reviewers

This is now additive rather than breaking: Wallet::finalize_psbt still returns bool

Intentionally not included here:

• changing Wallet::sign to return a richer result type
• any broader redesign beyond the PSBT finalization API surface

Changelog notice

• Add Wallet::try_finalize_psbt, which returns FinalizePsbtResult and exposes per-input finalization outcomes while preserving the existing Wallet::finalize_psbt API.

Checklists

All Submissions:

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

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[codecov]

Codecov Report

❌ Patch coverage is 91.42857% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.13%. Comparing base (fb7681a) to head (69966b6).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/wallet/mod.rs	91.42%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
+ Coverage   80.04%   80.13%   +0.09%     
==========================================
  Files          24       24              
  Lines        5336     5367      +31     
  Branches      242      242              
==========================================
+ Hits         4271     4301      +30     
- Misses        987      989       +2     
+ Partials       78       77       -1     

Flag	Coverage Δ
rust	80.13% <91.42%> (+0.09%)	⬆️

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[reez]

Updated per some good feedback from mammal