Allow copying session ID in Agentic Chat

What does this MR do and why?

It adds a button that allows copying an Agentic Chat's chat session ID. This is useful for troubleshooting purposes.

References

Allow users to see and copy workflow ID in Duo ... (gitlab-org/editor-extensions/gitlab-lsp#1398 - closed)

Screenshots or screen recordings

copy_clipboard_demo_monolith.mov

How to set up and validate locally

1. Start a new chat or open an existing one from the history panel.
2. A dropdown menu button will appear in the chat panel's header that allows copying the session ID. This is the same UI that we use in editors.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.