Update defaults to gpt-5.1 (#6652)

## Summary
- update documentation, example configs, and automation defaults to
reference gpt-5.1 / gpt-5.1-codex
- bump the CLI and core configuration defaults, model presets, and error
messaging to the new models while keeping the model-family/tool coverage
for legacy slugs
- refresh tests, fixtures, and TUI snapshots so they expect the upgraded
defaults

## Testing
- `cargo test -p codex-core
config::tests::test_precedence_fixture_with_gpt5_profile`


------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_6916c5b3c2b08321ace04ee38604fc6b)

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

@@ -364,7 +364,7 @@ async fn review_uses_custom_review_model_from_config() {
     // Choose a review model different from the main model; ensure it is used.
     let codex = new_conversation_for_server(&server, &codex_home, |cfg| {
         cfg.model = "gpt-4.1".to_string();
-        cfg.review_model = "gpt-5".to_string();
+        cfg.review_model = "gpt-5.1".to_string();
     })
     .await;
 
@@ -394,7 +394,7 @@ async fn review_uses_custom_review_model_from_config() {
     // Assert the request body model equals the configured review model
     let request = &server.received_requests().await.unwrap()[0];
     let body = request.body_json::<serde_json::Value>().unwrap();
-    assert_eq!(body["model"].as_str().unwrap(), "gpt-5");
+    assert_eq!(body["model"].as_str().unwrap(), "gpt-5.1");
 
     server.verify().await;
 }