feat: Add duo_agentic_chat_openai_gpt_5 feature flag

What does this MR do and why?

This MR introduces a new feature flag to later enable GPT5 in Agent Duo Chat.

References

Related MR in the AIGW to use GPT5 behind this feature flag - gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!3122 (merged).
Part of #560560.

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Before	After

How to set up and validate locally

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