Refactor: Updated TransactionError to use util::Expected and removed TransactionError:OK #35105

Summary

Similar to #32958 and motivated by #32958#pullrequestreview-3107029707

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

This makes the enum less confusing because it no longer includes OK.

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35105.

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline for information on the review process.

If your review is incorrectly listed, please copy-paste <code>&lt;!--meta-tag:bot-skip--&gt;</code> into the comment that the bot should ignore.

<!--174a7506f384e20aa4161008e828411d-->

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35511 (RFC: consensus: Make CAmount a class by hodlinator)
• #29278 (Wallet: Add maxfeerate wallet startup option 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.

<!--5faf32d7da4f0f540f40219e4f7537a3-->

Approach NACK

I do not think this is the best future proof way to accomplish the goal. In C++ 23 std::expected is introduced, which is a better way (imo) to do for explicit error handling. E.g link link2

Yes we are currently on C++ 20 but a "polyfill" std::expected is already introduced in the code as util::Expected<T, E>

This makes the upgrade path to C++23 an easy search and replace and is a better way to do explicit error handling / remove the ok from the errors.

@janb84 thanks for the review!

I agree and I pushed 592a833 to instead use util::Expected.

Feel free to give this another look and let me know if it seems fine to you

rebased and fixed conflicts to 1bcea55

<!--85328a0da195eb286784d51f73fa0af9-->

🚧 At least one of the CI tasks failed. _{Task iwyu: https://github.com/bitcoin/bitcoin/actions/runs/26795995122/job/78992358839} _{LLM reason (✨ experimental): CI failed because IWYU detected missing headers and intentionally failed after modifying src/node/interfaces.cpp (added #include <util/expected.h>).}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• 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.

Leave a comment here, if you need help tracking down a confusing failure.

</details>

Maybe turn into draft while CI is red?

Maybe turn into draft while CI is red?

Converted to draft, due to failing CI