[Bugfix] API stream returning two stops

[dylanwhawk]

Fixes #2266 by adapting #2267 to the current main

Example output:

data: {"id":"cmpl-64c697d392a840159142ea964ffe98cf","created":1710629390,"model":"facebook/opt-125m","choices":[{"index":0,"text":" and","logprobs":null,"finish_reason":null}],"usage":null}

data: {"id":"cmpl-64c697d392a840159142ea964ffe98cf","created":1710629390,"model":"facebook/opt-125m","choices":[{"index":0,"text":" I","logprobs":null,"finish_reason":"length"}],"usage":{"prompt_tokens":5,"total_tokens":10,"completion_tokens":5}}

data: [DONE]

[simon-mo]

please add a test for this behavior in tests enetrypoints

[dylanwhawk]

Test coverage added to test_completion_streaming and test_chat_streaming (cc @simon-mo)