Move frontend push of agentic_chat_ga feature flag to EE

What does this MR do and why?

!213639 (merged) added the EE agentic_chat_ga feature flag, but the FOSS code attempted to use it in lib/gitlab/gon_helper.rb. Move this into ee/lib/ee/gitlab/gon_helper.rb to fix this issue.

References

Relates to gitlab-org/quality/engineering-productivity/approved-mr-pipeline-incidents#10903 (closed)

How to set up and validate locally

FOSS_ONLY=1 bundle exec rspec ./spec/controllers/projects/commits_controller_spec.rb:253
bundle exec rspec ./spec/controllers/projects/commits_controller_spec.rb:253

MR acceptance checklist

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