add fast mode toggle (#13212)

- add a local Fast mode setting in codex-core (similar to how model id
is currently stored on disk locally)
- send `service_tier=priority` on requests when Fast is enabled
- add `/fast` in the TUI and persist it locally
- feature flag

codex-rs/core/tests/suite/image_rollout.rs

@@ -126,6 +126,7 @@ async fn copy_paste_local_image_persists_rollout_request_shape() -> anyhow::Resu
             model: session_model,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })
@@ -208,6 +209,7 @@ async fn drag_drop_image_persists_rollout_request_shape() -> anyhow::Result<()>
             model: session_model,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })