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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.15%. Comparing base (fb7681a) to head (e046adc).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
+ Coverage   80.04%   80.15%   +0.11%     
==========================================
  Files          24       24              
  Lines        5336     5367      +31     
  Branches      242      242              
==========================================
+ Hits         4271     4302      +31     
  Misses        987      987              
  Partials       78       78              

Flag	Coverage Δ
rust	80.15% <100.00%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[reez]

Updated per some good feedback from mammal