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/undo.rs

@@ -30,8 +30,8 @@ use pretty_assertions::assert_eq;
 async fn undo_harness() -> Result<TestCodexHarness> {
     TestCodexHarness::with_config(|config: &mut Config| {
         config.include_apply_patch_tool = true;
-        config.model = "gpt-5".to_string();
-        config.model_family = find_family_for_model("gpt-5").expect("gpt-5 is valid");
+        config.model = "gpt-5.1".to_string();
+        config.model_family = find_family_for_model("gpt-5.1").expect("gpt-5.1 is valid");
         config.features.enable(Feature::GhostCommit);
     })
     .await