[Usage Billing] Agentic Chat bills to the wrong namespace in a session across groups (member of both)

Agentic Chat currently uses bills based on the active namespace at the time of sending a message, but this contradicts the desired behavior defined in the feature request. We want the session’s origin namespace to own all billing, regardless of navigation.

Steps to reproduce:

1. Ensure you are a member of both CompanyA and CompanyB.
2. Start an Agentic Chat session while viewing Company A (Ultimate).
3. Navigate to Company B (also Ultimate, user is a member).
4. Continue sending messages in the same chat session.
5. Observe that messages sent while viewing Company B are billed to Company B.

Desired behavior (correct one):
All messages in a session should be billed to the namespace where the session started (Company A), even if the user navigates to Company B during the session.

Actual result:
Billing switches to Company B once the user navigates there.