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/model_overrides.rs

@@ -33,6 +33,7 @@ async fn override_turn_context_does_not_persist_when_config_exists() {
             model: Some("o3".to_string()),
             effort: Some(Some(ReasoningEffort::High)),
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })
@@ -69,6 +70,7 @@ async fn override_turn_context_does_not_create_config_file() {
             model: Some("o3".to_string()),
             effort: Some(Some(ReasoningEffort::Medium)),
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })