Workflow 5 Vitest follow-ups: install guidance, version guard, workflow refs (#4267)

Four follow-ups to the Workflow 5 Vitest path:

- Document that a v5 app has to install `@workflow/vitest@beta`, since
  `@workflow/vitest@latest` is still the 4.x line.
- Fail the test build up front when `@workflow/vitest` and the app resolve
  incompatible `@workflow/core` majors, with the install command that fixes
  it. Same-major differences warn. `WORKFLOW_VITEST_VERSION_CHECK=off` skips
  the check.
- Write the builder's `manifest.json` next to the test bundles and add
  `getWorkflowRef()` / `listWorkflowRefs()`, so a test can name a workflow
  instead of hand-writing the generated `workflow//...` id.
- Rewrite the mocking docs to match what `vi.mock()` actually reaches, and
  pin each case in the workbench.

Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
Co-authored-by: Peter Wielander <29887157+VaguelySerious@users.noreply.github.com>