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/client.rs

@@ -1128,6 +1128,7 @@ async fn user_turn_collaboration_mode_overrides_model_and_effort() -> anyhow::Re
                     .model_reasoning_summary
                     .unwrap_or(ReasoningSummary::Auto),
             ),
+            service_tier: None,
             collaboration_mode: Some(collaboration_mode),
             final_output_json_schema: None,
             personality: None,
@@ -1240,6 +1241,7 @@ async fn user_turn_explicit_reasoning_summary_overrides_model_catalog_default()
             model: session_configured.model,
             effort: None,
             summary: Some(ReasoningSummary::Concise),
+            service_tier: None,
             collaboration_mode: None,
             final_output_json_schema: None,
             personality: None,
@@ -1718,6 +1720,7 @@ async fn azure_responses_request_includes_store_and_reasoning_ids() {
             effort,
             summary.unwrap_or(ReasoningSummary::Auto),
             None,
+            None,
         )
         .await
         .expect("responses stream to start");