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/utils/tokenizer/src/lib.rs

@@ -158,7 +158,7 @@ mod tests {
     #[test]
     fn model_mapping_builds_tokenizer() -> Result<(), TokenizerError> {
         // Choose a long-standing model alias that maps to cl100k_base.
-        let tok = Tokenizer::for_model("gpt-5")?;
+        let tok = Tokenizer::for_model("gpt-5.1")?;
         let ids = tok.encode("ok", false);
         let back = tok.decode(&ids)?;
         assert_eq!(back, "ok");