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/exec/tests/suite/output_schema.rs

@@ -37,7 +37,7 @@ async fn exec_includes_output_schema_in_request() -> anyhow::Result<()> {
         .arg("--output-schema")
         .arg(&schema_path)
         .arg("-m")
-        .arg("gpt-5")
+        .arg("gpt-5.1")
         .arg("tell me a joke")
         .assert()
         .success();