feat: update default (#4076)

Changes:
- Default model and docs now use gpt-5-codex. 
- Disables the GPT-5 Codex NUX by default.
- Keeps presets available for API key users.

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

@@ -184,6 +184,7 @@ async fn prompt_tools_are_consistent_across_requests() {
 
     let conversation_manager =
         ConversationManager::with_auth(CodexAuth::from_api_key("Test API Key"));
+    let expected_instructions = config.model_family.base_instructions.clone();
     let codex = conversation_manager
         .new_conversation(config)
         .await
@@ -213,7 +214,6 @@ async fn prompt_tools_are_consistent_across_requests() {
     let requests = server.received_requests().await.unwrap();
     assert_eq!(requests.len(), 2, "expected two POST requests");
 
-    let expected_instructions: &str = include_str!("../../prompt.md");
     // our internal implementation is responsible for keeping tools in sync
     // with the OpenAI schema, so we just verify the tool presence here
     let expected_tools_names: &[&str] = &["shell", "update_plan", "apply_patch", "view_image"];