Payjoin ffi mutants

[benalleng]

Closes #1888

This adds both mutants coverage as well as coveralls tracking.

Pull Request Checklist

Please confirm the following before requesting review:

• I have disclosed my use of
  AI
  in the body of this PR.
• I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

[coveralls]

Coverage Report for CI Build 35651660534

Coverage decreased (-3.7%) to 82.961%

Details

• Coverage decreased (-3.7%) from the base build.
• Patch coverage: 11 uncovered changes across 2 files (349 of 360 lines covered, 96.94%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
payjoin-ffi/src/send/mod.rs	96	89	92.71%
payjoin-ffi/src/receive/mod.rs	164	160	97.56%
Total (4 files)	360	349	96.94%

Coverage Regressions

No coverage regressions found.

---

Coverage Stats

Relevant Lines:	18675
Covered Lines:	15493
Line Coverage:	82.96%
Coverage Strength:	303.68 hits per line

---

💛 - Coveralls

[benalleng]

I would like to get a little more raw coverage of the FFI src before marking it ready to review. I think I would like at least no coverage lost when adding this new workspace would be a nice bar to set.