agent_ui: Fix session equality check for reloading retained threads

[nathansobo]

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

[benbrandt]

The issue was Active thread session id could be a subagent or we could be in a loading/error state and the active thread isn't set yet. But we already know the session id, so we sholuld be using that instead

[benbrandt]

/cherry-pick preview