refactor(cli): consolidate relay logic in RelayManager

[va-an]

Relates to #1582

Done per the plan from #1582 (comment):

• Drop the Arc<Mutex<>> wrapper around RelayManager
• Put Arc<Mutex<>> on the failed_relays field instead
• Move the functions into &self methods on RelayManager

The original plan mentioned three functions, but App::unwrap_relay_or_else_fetch was turned into App::post_via_relay in #1628, so only two methods were moved here. It could be worth moving post_via_relay into RelayManager as well, but I'd suggest leaving that out of the scope of this PR.

Commands to verify the function moves (bash/zsh; strips the method's extra impl indent so only the signature/self-receiver changes show up). b28a1d8e is the master tip this branch is based on, pinned so the line ranges stay valid:

fetch_ohttp_keys:

diff -u \
  <(git show b28a1d8e:payjoin-cli/src/app/v2/ohttp.rs | sed -n '59,106p') \
  <(git show HEAD:payjoin-cli/src/app/v2/ohttp.rs      | sed -n '56,97p' | sed 's/^    //')

unwrap_ohttp_keys_or_else_fetch:

diff -u \
  <(git show b28a1d8e:payjoin-cli/src/app/v2/ohttp.rs | sed -n '49,57p') \
  <(git show HEAD:payjoin-cli/src/app/v2/ohttp.rs      | sed -n '49,54p' | sed 's/^    //')

Disclosure: code written by hand, Claude was used for review and discussion.

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[coveralls]

Coverage Report for CI Build 27272110652

Coverage increased (+0.05%) to 85.244%

Details

• Coverage increased (+0.05%) from the base build.
• Patch coverage: 30 uncovered changes across 2 files (10 of 40 lines covered, 25.0%).
• 1 coverage regression across 1 file.

Uncovered Changes

File	Changed	Covered	%
payjoin-cli/src/app/v2/ohttp.rs	38	9	23.68%
payjoin-cli/src/app/v2/mod.rs	2	1	50.0%

Coverage Regressions

1 previously-covered line in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
payjoin-cli/src/app/v2/ohttp.rs	1	38.0%

---

Coverage Stats

Relevant Lines:	14699
Covered Lines:	12530
Line Coverage:	85.24%
Coverage Strength:	371.39 hits per line

---

💛 - Coveralls

[benalleng]

TACK f130191

ran the cli with 5 active sessions and was able to see that once one relay hit the example.com relay it dropped it from the list and was not tried again during the lifetime of the resume process for any of the sessions