Multi threaded agentic chat on the web

solves: #556875 (closed)

What does this MR do and why?

This MR implements multi-threaded conversation support for GitLab Duo Agentic Chat, enabling users to create, manage, and switch between multiple chat threads within the same interface.

Key changes:

• Users can now view a list of previous conversations, select specific threads, and delete unwanted conversations
• Each thread maintains its own message history that persists across sessions
• Refactored chat logic into reusable utility classes (ApolloUtils, WorkflowUtils) for better maintainability
• Added thread list view with seamless switching between list and chat modes

Screenshots or screen recordings

Before	After

How to set up and validate locally

Follow the description in here: !197267 (merged)

MR acceptance checklist

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