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/json_result.rs

@@ -31,12 +31,12 @@ const SCHEMA: &str = r#"
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn codex_returns_json_result_for_gpt5() -> anyhow::Result<()> {
-    codex_returns_json_result("gpt-5".to_string()).await
+    codex_returns_json_result("gpt-5.1".to_string()).await
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn codex_returns_json_result_for_gpt5_codex() -> anyhow::Result<()> {
-    codex_returns_json_result("gpt-5-codex".to_string()).await
+    codex_returns_json_result("gpt-5.1-codex".to_string()).await
 }
 
 async fn codex_returns_json_result(model: String) -> anyhow::Result<()> {