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/grep_files.rs

@@ -11,7 +11,7 @@ use std::collections::HashSet;
 use std::path::Path;
 use std::process::Command as StdCommand;
 
-const MODEL_WITH_TOOL: &str = "test-gpt-5-codex";
+const MODEL_WITH_TOOL: &str = "test-gpt-5.1-codex";
 
 fn ripgrep_available() -> bool {
     StdCommand::new("rg")