Add core RelaySelector

[bc1cindy]

centralize OHTTP relay selection in payjoin::relay::RelaySelector so integrators share one policy instead of each diverging.

breaking: io::fetch_ohttp_keys(relay, dir) -> OhttpKeys becomes (relays, dir) -> (OhttpKeys, Url)

this is the follow-up referenced in #1695 (review) by @DanGould; directory selection stays in the manager; durable tracking is #1697

part of #1694 / part of #1586

disclosure: co-authored by Claude

[coveralls]

Coverage Report for CI Build 28468711613

Coverage increased (+0.2%) to 85.988%

Details

• Coverage increased (+0.2%) from the base build.
• Patch coverage: 19 uncovered changes across 3 files (123 of 142 lines covered, 86.62%).
• 2 coverage regressions across 2 files.

Uncovered Changes

File	Changed	Covered	%
payjoin-cli/src/app/v2/ohttp.rs	16	1	6.25%
payjoin/src/core/io.rs	57	54	94.74%
payjoin-cli/src/app/v2/mod.rs	6	5	83.33%
Total (5 files)	142	123	86.62%

Coverage Regressions

2 previously-covered lines in 2 files lost coverage.

File	Lines Losing Coverage	Coverage
payjoin-cli/src/app/v2/ohttp.rs	1	48.94%
payjoin/src/core/io.rs	1	77.07%

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Coverage Stats

Relevant Lines:	15372
Covered Lines:	13218
Line Coverage:	85.99%
Coverage Strength:	356.86 hits per line

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💛 - Coveralls

[zealsham]

Cack

one question, once a relay is marked as failed, its impossible to unfail except a total restart, Should we have time window for retryability ?

[bc1cindy]

thanks for the review! @zealsham

one question, once a relay is marked as failed, its impossible to unfail except a total restart, Should we have time window for retryability ?

RelaySelector only tracks failures in memory per operation. I opened #1697 for the failure-lifecycle policy, which covers exactly this "path back (re-test recovery)" question

keeping it out of this PR to stay focused on the selection refactor

[bc1cindy]

rebased onto master, integrated with #1695: relay selection moved out of MailroomManager into the lib RelaySelector