fix: guard against empty choices before accessing response.choices[0]

[qizwiz]

What

Add a guard before the response.choices[0] access in superagi/llms/openai.py.

Why

response.choices is an empty list — not an exception — when the provider applies content filtering or max_tokens is exceeded before any content is generated. Accessing choices[0] without a length check raises IndexError in production.

# before
content = response.choices[0].message["content"]

# after
if not response.choices or response.choices[0].message is None:
    raise ValueError("LLM returned empty or filtered response")
content = response.choices[0].message["content"]

Pre-existing test note

tests/unit_tests/llms/test_open_ai.py fails to import on the base branch due to openai.InvalidRequestError being removed in openai v1 (renamed to BadRequestError). This is unrelated to this change.

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