Fix agentConfig reset when reselecting same custom agent

What does this MR do and why?

Fixes a bug where agentConfig was incorrectly reset to null when selecting the same custom agent multiple times from the new chat dropdown.

Meant to solve: #579102

How to set up and validate locally

1. Enable custom agents in your GDK environment
2. Open Duo Agentic Chat
3. Select a custom (non-default!) agent from the dropdown
4. Send a message, verify the correct agentConfig being sent to the Backend
5. Click "New chat" and select the same custom agent again
6. Send another message
7. Verify the message still sends the correct agentConfig to the Backend (not null)

Screenshots or screen recordings

Before	After
2025-11-04_14-56-39	2025-11-04_14-51-14