Report structurally invalid workflows to users (#37116) (#37164)

Backport #37116 by @bircni

`model.ReadWorkflow` succeeds for YAML that is syntactically valid but
fails deeper parsing in `jobparser.Parse` (e.g. blank lines inside `run:
|` blocks cause a SetJob round-trip error). Add
`ValidateWorkflowContent` which runs the full `jobparser.Parse` to catch
these cases, and use it in the file view, the actions workflow list, and
the workflow detection loop so users see the error instead of silently
getting a 500 or a dropped workflow.

Fixes #37115

Signed-off-by: Nicolas <bircni@icloud.com>
Co-authored-by: Nicolas <bircni@icloud.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/repo/actions/actions.go

@@ -151,6 +151,11 @@ func prepareWorkflowTemplate(ctx *context.Context, commit *git.Commit) (workflow
 			workflows = append(workflows, workflow)
 			continue
 		}
+		if err := actions.ValidateWorkflowContent(content); err != nil {
+			workflow.ErrMsg = ctx.Locale.TrString("actions.runs.invalid_workflow_helper", err.Error())
+			workflows = append(workflows, workflow)
+			continue
+		}
 		workflow.Workflow = wf
 		// The workflow must contain at least one job without "needs". Otherwise, a deadlock will occur and no jobs will be able to run.
 		hasJobWithoutNeeds := false
@@ -315,6 +320,10 @@ func prepareWorkflowList(ctx *context.Context, workflows []WorkflowInfo) {
 			if !job.Status.IsWaiting() {
 				continue
 			}
+			if err := actions.ValidateWorkflowContent(job.WorkflowPayload); err != nil {
+				runErrors[run.ID] = ctx.Locale.TrString("actions.runs.invalid_workflow_helper", err.Error())
+				break
+			}
 			hasOnlineRunner := false
 			for _, runner := range runners {
 				if !runner.IsDisabled && runner.CanMatchLabels(job.RunsOn) {