create_chat-created chats: multi-chat pill is non-responsive

- Copilot Chat Extension Version:
- VS Code Version:
- OS Version:
- Feature (e.g. agent/edit/ask mode): agent mode (Agent Host / MCP Server integration)
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet):
- Logs:

Steps to Reproduce:

1. Have the Agent Host inject the MCP Server with the create_chat tool into an agent.
2. From the agent, call the create_chat tool (this tool creates a "chat", not a "session").
3. Open the Copilot Chat UI and locate the chat created by create_chat.
4. Click the multi-chat pill on the created chat.
5. Observe that clicking the multi-chat pill does nothing.

Additional details:
- create_chat appears to create a Chat object, but the UI expects multi-chats only inside Sessions; Chats do not contain multi-chats, so the multi-chat pill is non-responsive when used on chats created by this tool.
- This affects any agent integration that uses create_chat to programmatically create chats via the MCP Server/Agent Host.

Expected behavior:
- Clicking the multi-chat pill should open the multi-chat view or otherwise allow interacting with multi-chats for the created chat (or the create_chat tool should create a session if that's required for multi-chats).

Actual behavior:
- The multi-chat pill is non-responsive because Chats cannot host multi-chats, so nothing happens when clicked.

[View original Slack conversation](https://vscodeteam.slack.com/archives/C09MVCZKLP4/p1784317110378239?thread_ts=1784317110.378239&cid=C09MVCZKLP4)