Validates one task from a task list (initial or iteration): must not contain Output; only allowed field names; required keys must be present. Appends to issues when invalid. Call only after List.fill_names has been run (during parsing, before this); this method only reads the task name for issue messages and does not inject or set names.
Used by parse_task_list and parse_task_list_iteration so both paths share the same rules.
| t |
the task to validate (its task_data keys are checked) |