Folow up from Add Streaming MR

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What does this MR do and why?

This issue resolve the follow up from #423519 The purpose of this issue was to make sure that we don't "indefinitely" increase the length of that response_body but have a sensible limit. e.g. don't allow response_body to be larger than the maximum amount of Characters we allow in chat (which is 20k characters).

Folow up from Add Streaming MR

Changelog :other EE : true

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