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 36615674838

Coverage decreased (-2.1%) to 84.928%

Details

• Coverage decreased (-2.1%) from the base build.
• Patch coverage: 16 uncovered changes across 2 files (705 of 721 lines covered, 97.78%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
payjoin-ffi/src/receive/mod.rs	402	394	98.01%
payjoin-ffi/src/send/mod.rs	219	211	96.35%
Total (4 files)	721	705	97.78%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	19759
Covered Lines:	16781
Line Coverage:	84.93%
Coverage Strength:	290.61 hits per line

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💛 - 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.