Sessions list: Verify progress placement

[sandy081]

Verify progress ownership in the Agents Window Sessions list.

Verification:

1. Open a session with a main chat and at least one nested chat.
2. Start work in a nested chat while the session is expanded. Verify progress is shown on that chat row rather than redundantly on the parent session row.
3. Collapse the session while the nested chat is still running. Verify the hidden child progress is summarized on the parent row.
4. Start work owned by the main chat and verify its progress remains visible on the parent session row in both expanded and collapsed states.
5. Verify the accessible status text describes the same owner and state shown visually.

Implemented by #336445.