Fix LLM AI client not returning a HTTP response with 204 responses

What does this MR do and why?

Previously Llm::AiGateway::Client#request returned nil if the AI Gateway return a 204 No Content response. However, this makes it impossible to discern whether the request was successful or whether the server returned a 5xx error.

This was happening because run_retry_with_exponential_backoff returned nil if the body were blank. To fix this, return the HTTParty response even in this case.

References

None. I discovered this in development.

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