## Summary

Adds three tests that exercise session disassociation ("Session not
found" after reopening a thread). The third test **fails**, reproducing
the bug.

### Root cause

A retained `ConversationView` with a `thread_error` receives
`AgentServersUpdated`, which triggers `reset()` → `close_all_sessions` →
state transitions to `Loading`. When the user then reopens the same
session via `open_thread`, `load_agent_thread`'s `has_session` check
returns `false` (because the retained view is in `Loading` state, not
`Connected`), so a **second** `ConversationView` is created for the same
session. Both views' async load tasks complete and both transition to
`Connected` with the same session ID. When the retained view is later
cleaned up, its `on_release` → `close_all_sessions` removes the session
from the connection, leaving the active view stranded with a dead
session.

### Tests

| Test | Status | What it verifies |
|------|--------|-----------------|
| `test_open_thread_on_visible_session_does_not_disassociate_it` | ✅
Pass | `load_agent_thread` short-circuits when session is already active
|
| `test_retained_thread_cleanup_does_not_disassociate_reopened_session`
| ✅ Pass | Eviction + reopen cycle works (close-then-reload) |
| `test_retained_thread_reset_race_disassociates_session` | ❌ **Fail** |
Reproduces the race described above |

cc @bennetbo @ConradIrwin

Release Notes:

- N/A

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>