Add require_approval preflight check to @task.llm_schema_compare (#71688)

@task.llm, @task.llm_branch, and @task.llm_sql all reject a non-string
Sequence[UserContent] prompt combined with require_approval=True before
the LLM runs, with an error naming the decorator the caller actually used.
@task.llm_schema_compare was missing this decorator-level check, so the
same misuse instead fell through to the underlying operator's own guard --
which names the internal _LLMSchemaCompareDecoratedOperator class rather
than the decorator, confusing anyone who only ever wrote
@task.llm_schema_compare.