Re-Integrate Multi-Threaded DuoChat into the GitLab Monolith

This commit:
- Implements multi-threaded DuoChat integration
- Updates all GraphQL queries required for the multi-threaded architecture.
- Ensures seamless fallback to single-threaded mode when duo_chat_multi_thread is disabled.
It was initially merged in:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/179708


Reverted to some breaking-changes in the API
that are resolved.

EE: true

Merged-by: Jannik Lehmann <jlehmann@gitlab.com>
Approved-by: Lindsey Shelton <lshelton@gitlab.com>
Approved-by: Andrew Fontaine <afontaine@gitlab.com>
Reviewed-by: Andrew Fontaine <afontaine@gitlab.com>
Reviewed-by: Jannik Lehmann <jlehmann@gitlab.com>