Streaming with OpenAI Responses API breaks on unexpected post-completion events

## Problem OpenAI's Responses API has started emitting a `response.rate_limits.updated` event **after** the `response.completed` event during streaming. This causes `ChatOpenAI()` to crash with: ``` AttributeError: 'NoneType' object has no attribute 'output' ``` ## Minimal reproducible example ```python from chatlas import ChatOpenAI chat = ChatOpenAI() chat.chat("What is 1 + 1?") # AttributeError: 'NoneType' object has no attribute 'output' ``` ## Root cause In `OpenAIProvider.stream_merge_chunks()`, the method correctly captures the `Response` object when it sees `response.completed`, but its fallthrough path for unhandled event types returns `cast(Response, None)`: ```python def stream_merge_chunks(self, completion, chunk): if chunk.type == "response.completed": return chunk.response elif chunk.type == "response.failed": ... elif chunk.type == "error": ... # This overwrites a valid Response with None! return cast(Response, None) ``` When `response.rate_limits.updated` arrives after `response.completed`, the valid `Response` is overwritten with `None`, which then gets passed to `stream_turn()` → `_response_as_turn()`, causing the crash. ## Context This appears to be a server-side change from OpenAI (the new event is emitted regardless of SDK version). The event appeared during testing on `openai>=2.30.0` but may roll out intermittently. ## Fix Change the fallthrough to `return completion`, preserving whatever value has already been accumulated. This also makes the code resilient to any future unknown event types.