Clarify default behavior of Knowledge Base search in frontend AI chat

Is your feature request related to a problem? Please describe.
In the frontend AI chat interface, when selecting "Knowledge Base Search", the default behavior is to search globally across all knowledge bases. Users can then toggle a switch to select specific knowledge bases. However, there is no clear indication in the UI that the initial search is global, which can lead to confusion for users who might expect it to search within a specific context or not search at all until a knowledge base is selected. This ambiguity can negatively impact the user experience.

Describe the solution you'd like
Add a clear explanation or visual indicator in the frontend UI to inform users that the default behavior of "Knowledge Base Search" is to search globally. This could be a small text label next to the search option, a tooltip, or a change in the UI state before a specific knowledge base is selected. The goal is to make it immediately clear to the user what the search scope is.

Describe alternatives you've considered
One alternative could be to change the default behavior to not perform a search until the user explicitly selects a knowledge base. However, this might not be the desired user flow. Another alternative is to rely solely on documentation, but this is less effective than in-context UI clarification.

Additional context
This issue is observed in the AI chat feature on the frontend when the "Knowledge Base Search" option is active.

Start in the frontend AI chat entry point where the "Knowledge Base Search" option is rendered and inspect how its default global scope is represented. Add an in-context explanation or visual indicator for the global search state, then verify that the scope is clear before and after selecting specific knowledge bases.