When reopening a thread that is already visible in the agent panel,
`open_thread` could recreate the session instead of reusing the existing
visible thread. That left the UI holding a thread whose backend session
could be closed out from under it.

This change routes `AgentPanel::open_thread` through
`load_agent_thread`, which already handles the right behavior for
existing sessions:
- reuse the active thread when it already matches
- promote a retained thread when it matches
- only load when the session is not already present

The regression coverage is now a focused behavioral test at the
`AgentPanel::open_thread` boundary that verifies reopening an
already-visible session keeps the thread usable.

cc @bennetbo

Release Notes:

- Fixed reopening an already-visible agent thread so it reuses the
existing session instead of creating a broken duplicate.