Forgejo Actions allows variables in `jobs.<job_id>.runs-on`. However, the action list [skips checking whether a suitable runner](c3412d0579/routers/web/repo/actions/actions.go (L114-L148)) is available if an expression contains variables. That hampers a user's ability to figure out whether an expression was evaluated correctly and why a job might not be picked up by an available runner.
This PR adds the ability to surface more complex and additional diagnostic information on the action view screen. Previously, only a job's status (waiting, running, ...) was displayed. Now, extended messages like "Waiting for a runner with the following labels: docker, trixie" are displayed with the possibility to show multiple messages simultaneously.
How it looked before:

How it looks after updating Forgejo without reloading the window:

How it looks afterwards with a single label:

How it looks afterwards with multiple labels:

## Checklist
The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).
### Tests
- I added test coverage for Go changes...
- [x] in their respective `*_test.go` for unit tests.
- [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
- [x] in `web_src/js/*.test.js` if it can be unit tested.
- [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).
### Documentation
- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.
### Release notes
- [ ] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.
<!--start release-notes-assistant-->
## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Features
- [PR](https://codeberg.org/forgejo/forgejo/pulls/9966): <!--number 9966 --><!--line 0 --><!--description ZGlzcGxheSBkZXRhaWxlZCBhY3Rpb24gcnVuIGRpYWdub3N0aWNz-->display detailed action run diagnostics<!--description-->
<!--end release-notes-assistant-->
Co-authored-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9966
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: Andreas Ahlenstorf <andreas@ahlenstorf.ch>
Co-committed-by: Andreas Ahlenstorf <andreas@ahlenstorf.ch>