Avoid closing the choice when an exception happens during its generation

[adubovik]

Currently, a choice is closed even when an exception was thrown during its generation:

ai-dial-sdk/aidial_sdk/chat_completion/choice.py

Lines 55 to 62 in 8be932c

	def __exit__(
	self,
	exc_type: Optional[Type[BaseException]],
	exc: Optional[BaseException],
	traceback: Optional[TracebackType],
	) -> Optional[bool]:
	self.close()
	return False

This leads to confusing streams of chunks like this one:

data: {"choices":[{"index":0,"finish_reason":null,"delta":{"role":"assistant"}}],"usage":null,"id":"7b8ef0df-26a6-4382-866a-98cf0a36bd1c","created":1732815811,"object":"chat.completion.chunk"}
data: {"choices":[{"index":0,"finish_reason":"stop","delta":{}}],"usage":null,"id":"7b8ef0df-26a6-4382-866a-98cf0a36bd1c","created":1732815811,"object":"chat.completion.chunk"}
data: {"error":{"message":"Something went wrong!","type":"internal_server_error"}}
data: [DONE]

This creates a false impression that the generation of a choice was successful, but something went wrong later, after the choice was generated.

Note that the stages are closed with status=failed in cases of an exception:

ai-dial-sdk/aidial_sdk/chat_completion/stage.py

Lines 48 to 60 in 8be932c

	def __exit__(
	self,
	exc_type: Optional[Type[BaseException]],
	exc: Optional[BaseException],
	traceback: Optional[TracebackType],
	) -> Optional[bool]:
	if not exc:
	if not self._closed:
	self.close(Status.COMPLETED)
	else:
	self.close(Status.FAILED)
	return False

for which this no analog in the choice API.