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/client.rs

@@ -822,7 +822,7 @@ async fn token_count_includes_rate_limits_snapshot() {
                     "reasoning_output_tokens": 0,
                     "total_tokens": 123
                 },
-                // Default model is gpt-5 in tests → 272000 context window
+                // Default model is gpt-5-codex in tests → 272000 context window
                 "model_context_window": 272000
             },
             "rate_limits": {

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

@@ -133,7 +133,7 @@ async fn compact_resume_and_fork_preserve_model_history_view() {
         .to_string();
     let user_turn_1 = json!(
     {
-      "model": "gpt-5",
+      "model": "gpt-5-codex",
       "instructions": prompt,
       "input": [
         {
@@ -182,7 +182,7 @@ async fn compact_resume_and_fork_preserve_model_history_view() {
     });
     let compact_1 = json!(
     {
-      "model": "gpt-5",
+      "model": "gpt-5-codex",
       "instructions": "You have exceeded the maximum number of tokens, please stop coding and instead write a short memento message for the next agent. Your note should:
 - Summarize what you finished and what still needs work. If there was a recent update_plan call, repeat its steps verbatim.
 - List outstanding TODOs with file paths / line numbers so they're easy to find.
@@ -255,7 +255,7 @@ async fn compact_resume_and_fork_preserve_model_history_view() {
     });
     let user_turn_2_after_compact = json!(
     {
-      "model": "gpt-5",
+      "model": "gpt-5-codex",
       "instructions": prompt,
       "input": [
         {
@@ -320,7 +320,7 @@ SUMMARY_ONLY_CONTEXT"
     });
     let usert_turn_3_after_resume = json!(
     {
-      "model": "gpt-5",
+      "model": "gpt-5-codex",
       "instructions": prompt,
       "input": [
         {
@@ -405,7 +405,7 @@ SUMMARY_ONLY_CONTEXT"
     });
     let user_turn_3_after_fork = json!(
     {
-      "model": "gpt-5",
+      "model": "gpt-5-codex",
       "instructions": prompt,
       "input": [
         {

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"];