Call for testers: v5.0.0-beta.1 (delegates to game-ci/cli's extracted logic)

We just cut [`v5.0.0-beta.1`](https://github.com/game-ci/unity-test-runner/releases/tag/v5.0.0-beta.1) — a preview of the next major version of `unity-test-runner` (see [#310](https://github.com/game-ci/unity-test-runner/pull/310) for the full technical writeup). It's tagged separately from `@v4`/`@v3` — nobody's existing workflow is affected by this.

Note this one's architecturally a bit different from `unity-builder`/`unity-activate`'s pre-releases: those now shell out to the `game-ci` CLI binary directly. This action can't do that yet — `game-ci/cli` doesn't have a Docker-based `test` command yet ([cli#71](https://github.com/game-ci/cli/issues/71) tracks that). Instead, this action's own logic has been extracted into `game-ci/cli`'s in-repo `plugins/unity/` workspace and this action re-exports it as a pinned git dependency. Meant to be a drop-in replacement either way — nothing new to configure.

## How to try it

```yaml
- uses: game-ci/unity-test-runner@v5.0.0-beta.1
with:
projectPath: test-project
```

Everything else — inputs, secrets, artifacts, coverage, GitHub Checks reporting — stays the same as `@v4`.

## Known context

- Depends on a pinned commit of `game-ci/cli`'s `plugins/unity/` workspace, currently up to date with `cli`'s `main` (including a recent fix for intermittent Windows Docker-launch failures — [#314](https://github.com/game-ci/unity-test-runner/issues/314)).

Full details in the [release notes](https://github.com/game-ci/unity-test-runner/releases/tag/v5.0.0-beta.1).

## What to report here

- Did your test run succeed/fail, and how did that compare to `@v4`?
- Unity version, test mode (edit/play/standalone/package), and runner OS you tested with.
- Anything that felt different, even something minor.

Thanks for helping get this solid before it ships as the default. 🙏

---

### Ready-to-post Discord announcement

If you're active in the [GameCI Discord](https://discord.gg/game-ci) and want to help spread the word, here's a copy-pasteable version for the announcement/help channel:

```
Hey folks! We just cut a pre-release of the next major `unity-test-runner` version — v5.0.0-beta.1. Its test/build logic now lives in game-ci/cli's plugins/unity workspace instead of this repo directly (same underlying behavior, just re-exported from a pinned dependency). Tagged separately from @v4, so it won't touch anyone's existing workflows.

If you've got a spare few minutes, we'd love for you to try it before the real v5 ships:

- uses: game-ci/unity-test-runner@v5.0.0-beta.1
with:
projectPath: test-project

Everything else stays the same as @v4 - just swap the tag, nothing new to configure.

Report back in the tracking issue: https://github.com/game-ci/unity-test-runner/issues/316

Thanks for helping kick the tires! 🙏
```

Start with the v5.0.0-beta.1 workflow example in the issue and run it against a Unity project, then repeat with @v4 for comparison. Record whether the run succeeds, the Unity version, test mode, runner OS, and any differences or minor issues; that report is the definition of done.

Written by the indexing model from the issue text.