feat: support GPT5 in agentic Duo Chat under the feature flag

Alexander Chueshevrequested to merge
ac/use-gpt5-agentic-duo-chat into main

What does this merge request do and why?

This MR enables GPT5 in Duo Chat behind the feature flag (gitlab-org/gitlab!200789 (merged)).

Closes gitlab-org/gitlab#560560

How to set up and validate locally

  1. Enable the feature flag in GDK by running: Feature.enable(:duo_agentic_chat_openai_gpt_5).
  2. Set up OPENAI_API_KEY in .env along with other required env variables listed in the doc.
  3. Run poetry run duo-workflow-service
  4. Open the VSCode extension to test the new model in the Agentic Duo Chat UI.

Merge request checklist

  • Tests added for new functionality. If not, please raise an issue to follow up.
  • Documentation added/updated, if needed.
Edited by Alexander Chueshev

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