Refactor: Updated signMessage to use util::Expected and removed SigningResult::OK #35201

Summary

Similar to #32958 and #35105.

We now are using util::Expected and removed SigningResult::OK

This makes the enum less confusing because now it no longer includes OK and is now a pure error enum.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35043 (refactor: Properly return from ThreadSafeQuestion signal + btcsignals follow-ups by maflcko)
• #34806 (refactor: logging: Various API improvements by ajtowns)
• #34617 (fees: wallet: remove block policy fee estimator internals from wallet by ismaelsadeeq)
• #34075 (fees: Introduce Mempool Based Fee Estimation to reduce overestimation by ismaelsadeeq)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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🚧 At least one of the CI tasks failed. _{Task test ancestor commits: https://github.com/bitcoin/bitcoin/actions/runs/25280307633/job/74116469483} _{LLM reason (✨ experimental): CI failed due to a Clang -Werror build error: the switch in signverifymessagedialog.cpp doesn’t handle the OK enum value.}

<details><summary>Hints</summary>

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• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

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</details>

Looks like the CI failure is fixed with this PR #35202 will rebase onto master once it's merged to get the CI passing

rebased to e14b3ff

The CI should pass now