Duo Chat: notify default namespace not specified

Duo Agentic Chat will be available to users outside the namespace context, but that requires default namespace being selected.

This is less of an issue for now, as we always have classic chat as fallback, which does not require default namespace.

However as we slowly move to agentic chat, and the chat panel being always present, it is beneficial for user to know the need to select a default namespace, when the user attempts to open Duo Agentic Chat.

Proposal

We can conditionally display the message when Agentic Chat is not available to let them know to go and set default namespaces. This is probably easier to implement than an in-chat message, since the chat logic often rely on the presence of namespace.