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.

MR acceptance checklist

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assigned to @stanhu

added pipelinetier-1 label

changed milestone to %17.6

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	2 Warnings
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	This MR changes code in ee/, but its Changelog commit is missing the EE: true trailer. Consider adding it to your Changelog commits.

The following files contain potentially non-compliant AI logging:

• ee/lib/gitlab/llm/vertex_ai/client.rb

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• 13a83af3 - Fix LLM AI client not returning a HTTP response with 204 responses

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• 44029c92 - Fix LLM AI client not returning a HTTP response with 204 responses

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• 1fb48fb0 - Fix LLM AI client not returning a HTTP response with 204 responses

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requested review from @mksionek, @dbiryukov, and @maddievn