### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### The Feature

Add an `aclose()` coroutine to `litellm.responses.streaming_iterator.BaseResponsesAPIStreamingIterator` (and by extension `ResponsesAPIStreamingIterator`) so that consumers can deterministically release the underlying `httpx.Response` when the stream is abandoned mid-iteration — matching the pattern introduced for chat-completions streaming in [PR #21213](https://github.com/BerriAI/litellm/pull/21213)).

Proposed implementation (mirrors `CustomStreamWrapper.aclose` from #21213):

```python class BaseResponsesAPIStreamingIterator: async def aclose(self) -> None: response = getattr(self, "response", None) if response is None: return with anyio.CancelScope(shield=True): try: await response.aclose() except BaseException as e: verbose_logger.debug( "ResponsesAPIStreamingIterator.aclose: error closing response: %s", e ) finally: self.finished = True ```

### Motivation, pitch

PR [#21213](https:/…