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/codex-api/tests/clients.rs

@@ -265,6 +265,7 @@ async fn streaming_client_retries_on_transport_error() -> Result<()> {
         store: false,
         stream: true,
         include: Vec::new(),
+        service_tier: None,
         prompt_cache_key: None,
         text: None,
     };
@@ -306,6 +307,7 @@ async fn azure_default_store_attaches_ids_and_headers() -> Result<()> {
         store: true,
         stream: true,
         include: Vec::new(),
+        service_tier: None,
         prompt_cache_key: None,
         text: None,
     };