fix(descriptor): propagate threshold errors instead of panicking

[busayo-OD]

Fixes #532

Description

thresh_vec, multi_vec, and multi_a_vec used .expect(...) on Threshold::new, causing invalid runtime-supplied thresholds to panic instead of returning through the macros' usual Result<_, DescriptorError>.

For thresh_vec, the error is propagated through its existing Result chain. For multi_vec and multi_a_vec, keys::make_multi now accepts a closure returning Result<Terminal<...>, DescriptorError>, matching the existing make_sortedmulti pattern and removing the need for .expect(...).

Notes to the reviewers

miniscript already provides Error::Threshold for invalid thresholds, and DescriptorError already wraps miniscript::Error, so I reused the existing error path rather than introducing a new variant. Happy to switch to a dedicated variant if you'd rather keep threshold failures more explicit.

Changelog notice

`keys::make_multi`'s closure now returns `Result<Terminal<DescriptorPublicKey, Ctx>, DescriptorError>` instead of `Terminal<...>` directly.

Checklists

All Submissions

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

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 81.84%. Comparing base (e13e8a0) to head (d031217).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #533   +/-   ##
=======================================
  Coverage   81.84%   81.84%           
=======================================
  Files          25       25           
  Lines        6487     6490    +3     
  Branches      296      296           
=======================================
+ Hits         5309     5312    +3     
  Misses       1078     1078           
  Partials      100      100           

Flag	Coverage Δ
rust	81.84% <100.00%> (+<0.01%)	⬆️

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

☔ View full report in Codecov by Harness.
📢 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.

[Arowolokehinde]

I pulled this branch and ran all three tests locally. The fix correctly stops the fragment! macro from panicking on invalid thresholds and returns Err(DescriptorError::Miniscript(miniscript::Error::Threshold(_))) instead, routing errors through the existing Result channel rather than bypassing it with .expect().