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)

docs/example-config.md

@@ -19,11 +19,11 @@ Use this example configuration as a starting point. For an explanation of each f
 ################################################################################
 
 # Primary model used by Codex. Default differs by OS; non-Windows defaults here.
-# Linux/macOS default: "gpt-5-codex"; Windows default: "gpt-5".
-model = "gpt-5-codex"
+# Linux/macOS default: "gpt-5.1-codex"; Windows default: "gpt-5.1".
+model = "gpt-5.1-codex"
 
-# Model used by the /review feature (code reviews). Default: "gpt-5-codex".
-review_model = "gpt-5-codex"
+# Model used by the /review feature (code reviews). Default: "gpt-5.1-codex".
+review_model = "gpt-5.1-codex"
 
 # Provider id selected from [model_providers]. Default: "openai".
 model_provider = "openai"
@@ -315,7 +315,7 @@ mcp_oauth_credentials_store = "auto"
 [profiles]
 
 # [profiles.default]
-# model = "gpt-5-codex"
+# model = "gpt-5.1-codex"
 # model_provider = "openai"
 # approval_policy = "on-request"
 # sandbox_mode = "read-only"