New knowledge sources not showing up when creating assistants

Summary:

A knowledge source created in AI Studio is functional - it can be attached to a session and the model can read and interact with it. However, when creating an AI Assistant and attempting to attach that same knowledge base, it does not appear in the knowledge source list.

Steps to Reproduce:

1. Create a knowledge source in AI Studio (creation succeeds).

2. Attach it to a session - confirm the model can read and interact with it.

3. Start creating an AI Assistant and open the knowledge source list to attach it.

Actual Result:

• The knowledge source does not appear in the list, so it cannot be attached to the AI Assistant.

• The source itself works correctly when used in a session, so the problem is not with the source's content or availability.

Expected Result:

Any successfully created knowledge source should be selectable when configuring an AI Assistant.