Allow Duo Core user to still use DAP code review

What does this MR do and why?

We are about to remove Duo Core from classic chat's scope. To still allow user to use Duo Code Review (by mentioning @GitlabDuo when replying to a MR), for now we temporarily revert the unit_primitive to "duo_chat".

References

Screenshots or screen recordings

The first reply is before the fix, and second reply is after the fix:

Screenshot_2026-04-10_at_14-05-25_MR___19___19____Merge_requests___gitlab-duo-enterprise___merge-trains-9e94bf1b-9eb2-4241-9da5-eb7c61f8a46d___GitLab

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Related to #581229

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