Codebase Search Tool for Agentic Chat - support explicit project inclusion

Context

As part of supporting "Chat with your codebase" in Agentic Chat, we need to introduce the Codebase Search Tool into it.

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

1. the implicitly included project: the active project in the user's workspace - #1287 (moved)
2. explicitly included projects: projects included through /include <repository> - THIS ISSUE

#1287 (moved) will introduce the Codebase Search Tool with support for the active project in the workspace. In this issue, we will update the tool to support explicitly included projects.

References

Add the Codebase Search Tool to Agentic Chat - ... (#1287 - moved)

Proposal

Expand the implementation in Add the Codebase Search Tool to Agentic Chat - ... (#1287 - moved) to cover multiple projects