This PR fixes issue #3185
The code calls `await event_gen.aclose()` but OpenAI's `AsyncStream`
doesn't have an `aclose()` method - it has `close()` (which is async).
when clients cancel streaming requests, the server tries to clean up
with:
```python
await event_gen.aclose() # ❌ AsyncStream doesn't have aclose()!
```
But `AsyncStream` has never had a public `aclose()` method. The error
message literally tells us:
```
AttributeError: 'AsyncStream' object has no attribute 'aclose'. Did you mean: 'close'?
^^^^^^^^
```
## Verification
* Reproduction script
[`reproduce_issue_3185.sh`](https://gist.github.com/r-bit-rry/dea4f8fbb81c446f5db50ea7abd6379b)
can be used to verify the fix.
* Manual checks, validation against original OpenAI library code
