fix(actions): reject jobs without runs-on (#39480)

Align job and `runs-on` validation with github.com, as implemented by
the parser in https://github.com/actions/runner. A job without `runs-on`
could be claimed by any runner, so a job meant for a container could run
on the host.

- Jobs without `runs-on` fail with `Required property is missing:
runs-on`, called workflows included
- Unknown job keys and callers (`uses:`) mixed with steps-only keys like
`runs-on` are rejected
- Empty, null and nested `runs-on` values are rejected
- A `runs-on` evaluating to such a value fails only that job
- Called workflows are validated at run creation, an invalid one fails
the run as an invalid workflow file
- Zero labels (`runs-on: []` or `{}`) never match a runner, including
jobs queued before upgrading

<img width="960" alt="image"
src="https://github.com/user-attachments/assets/e746ce5a-b711-4e8b-aab8-81336ff53d86"
/>

**Behavior Change:** workflows that omit `runs-on`, use unknown job keys
or mix `uses` with `runs-on` stop running until fixed.

---------

Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

models/actions/runner.go

@@ -200,7 +200,7 @@ func (r *ActionRunner) GenerateAndFillToken() {
 // CanMatchLabels checks whether the runner's labels can match a job's "runs-on"
 // See https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#jobsjob_idruns-on
 func (r *ActionRunner) CanMatchLabels(jobRunsOn []string) bool {
-	return !slices.ContainsFunc(jobRunsOn, func(label string) bool { return !util.SliceContainsString(r.AgentLabels, label, true) })
+	return len(jobRunsOn) > 0 && !slices.ContainsFunc(jobRunsOn, func(label string) bool { return !util.SliceContainsString(r.AgentLabels, label, true) })
 }
 
 func init() {

models/actions/runner_test.go

@@ -86,4 +86,5 @@ func TestCanMatchLabelsCaseInsensitive(t *testing.T) {
 	runner := &ActionRunner{AgentLabels: []string{"self-hosted", "Linux", "X64"}}
 	assert.True(t, runner.CanMatchLabels([]string{"SELF-HOSTED", "linux"}))
 	assert.False(t, runner.CanMatchLabels([]string{"linux", "arm64"}))
+	assert.False(t, runner.CanMatchLabels(nil))
 }