Report structurally invalid workflows to users (#37116)

`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: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/actions/workflows.go

@@ -103,10 +103,20 @@ func GetEventsFromContent(content []byte) ([]*jobparser.Event, error) {
 	if err != nil {
 		return nil, err
 	}
+	if err := ValidateWorkflowContent(content); err != nil {
+		return nil, err
+	}
 
 	return events, nil
 }
 
+// ValidateWorkflowContent catches structural errors (e.g. blank lines in run: | blocks)
+// that model.ReadWorkflow alone does not detect.
+func ValidateWorkflowContent(content []byte) error {
+	_, err := jobparser.Parse(content)
+	return err
+}
+
 func DetectWorkflows(
 	gitRepo *git.Repository,
 	commit *git.Commit,