refactor(ai-proxy) clean up code and fix gzip response

[fffonion]

Summary

• move shared ctx usage of ai plugins to use a proper API to make typo more obvious to be catched
• only send gzipped response if client has Accept-Encoding that accepts gzip
• style cleanup

Checklist

• A changelog file has been added to CHANGELOG/unreleased/kong or adding skip-changelog label on PR if unnecessary. README.md

Issue reference

AG-39

[liverpool8056]

Just a tiny suggestion: we can just return kong.ctx.shared.llm_disable_ai_proxy_response_transform since we ensure it is a bool itself by a dedicated function.

[fffonion]

Good point, we can fix in the next round of refactor.

[team-gateway-bot]

Cherry-pick failed for master, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git remote add upstream https://github.com/kong/kong-ee
git fetch upstream master
git worktree add -d .worktree/cherry-pick-13155-to-master-to-upstream upstream/master
cd .worktree/cherry-pick-13155-to-master-to-upstream
git checkout -b cherry-pick-13155-to-master-to-upstream
ancref=$(git merge-base c882de932965a21d89100b1f1c1421b627460c4a a69eebff9ee61046c5f5d41d11813f3c9f390757)
git cherry-pick -x $ancref..a69eebff9ee61046c5f5d41d11813f3c9f390757