test: add csharp integration test

[ValeraFinebits]

Add C# integration tests for Payjoin FFI, similar to other languages.

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]

Pull Request Test Coverage Report for Build 22256960072

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.444%

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Totals
Change from base Build 22242311720:	0.0%
Covered Lines:	10594
Relevant Lines:	12850

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💛 - Coveralls

[spacebear21]

ACK 8fe975f, thanks for the prompt follow up!

I don't know C# at all so can't speak to the quality of the code but it seems reasonable to me and the CI confirms the newly added tests pass.