Why

After turning Fast mode off in the TUI, returning from a side thread could make Fast appear again in the main chat widget. The opt-out itself was still persisted; the display was being rebuilt from stale cached ThreadSessionState data, which made it look like Fast had been re-enabled.

Fixes #23104.

What changed

• Keep the active thread's cached service_tier in sync whenever the user persists a service-tier selection.
• Update both the primary-thread snapshot and the thread event store so restored TUI state reflects the current tier.
• Add a focused regression test for clearing a cached Fast tier.

Manual repro

1. Start a TUI session where Fast is enabled by default.
2. Run /fast and turn Fast mode off. Confirm Fast disappears from the chat widget display.
3. Re-enter thread navigation via either path:
   • Run /side test, then return to the main thread.
   • Run /agent, enter a child thread, then return to the main thread.
4. Before this fix, Fast reappears in the main chat widget display even though the opt-out was already persisted.
5. After this fix, Fast stays cleared.

Verification

• cargo test -p codex-tui app::thread_session_state::tests::service_tier_sync_updates_active_cached_session -- --exact