Fix Duo Agentic Chat subheader alignment

What does this MR do and why?

Fixes vertical alignment issues in the Duo Agentic Chat subheader:

• Moves the session-id dropdown (the #header slot in duo_agentic_chat_state_manager.vue) out of its absolutely-positioned wrapper and into the chat header's right-side flex container. The previous gl-absolute gl-right-5 gl-top-10 gl-pt-2 positioning required hard-coded offsets and didn't reliably align with the subheader content.
• Adds gl-items-center to the inner avatar/title block in duo_chat_header.vue so the 32px avatar stays vertically centered with the title and agent handler text instead of sticking to the top of a stretched flex row.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open Duo Agentic Chat.
2. Start a new chat and confirm the empty state subheader still renders as before.
3. Open an existing thread and confirm the avatar, title/agent handler, and session-id dropdown are all vertically aligned in the middle of the subheader row.

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