[Agentic Chat] Support multiple projects included through /include repository

Context

As part of supporting "Chat with your codebase" in Agentic Chat, we are introducing the Codebase Search Tool into it (#569624 (closed)).

This search tool must be able to support codebase search on:

1. A single project - #569624 (closed) - ✅
   • The active project in the user's workspace
   • A single project included through /include repository
   • A single project specified in the user's question
2. Multiple projects - THIS ISSUE
   • Multiple projects specified in the user's question - no changes needed
   • Multiple projects selected through /include repository

References

Epic: &18193

Proposal

TBA