Fall back to first candidate if avoid_uih fails

[DanGould]

try_preserving_privacy is about a best-effort attempt, not a guaranteed success. This implementation is what was already implied by the docstring. So it's a fix.

Making avoid_uih and select_first_candidate public later can be public to allow for more granular downstream control.

[DanGould]

It's not yet clear to me whether we want to expose the private coin selection methods in-place as public, or whether we want to make public pure functions that a sophisticated downstream receiver can implement themselves. I think the latter is a whole lot cleaner but is also more code to maintain. It might be a good place for us to start addressing the coin selection problem in general in this repo.

Using Psbt V2 Inputs would make the interface a whole lot cleaner instead of using our custom InputPair type.

[0xBEEFCAF3]

utACK b88511c

[0xBEEFCAF3]

Nit: specify "two-output transaction" instead of "multi-output transaction".

[0xBEEFCAF3]

utACK 1201492

[spacebear21]

Not directly related to this change, I noticed select_first_candidate returns a InternalSelectionError::NotFound if there are no candidates - that should be InternalSelectionError::Empty instead.

[coveralls]

Pull Request Test Coverage Report for Build 13294028995

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.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 6 of 7 (85.71%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on only-try-preserving-privacy at 79.331%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin/src/receive/error.rs	0	1	0.0%

Totals
Change from base Build 13251464274:	79.3%
Covered Lines:	4030
Relevant Lines:	5080

---

💛 - Coveralls