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/prompt_caching.rs

@@ -160,7 +160,7 @@ async fn prompt_tools_are_consistent_across_requests() -> anyhow::Result<()> {
     // with the OpenAI schema, so we just verify the tool presence here
     let tools_by_model: HashMap<&'static str, Vec<&'static str>> = HashMap::from([
         (
-            "gpt-5",
+            "gpt-5.1",
             vec![
                 "shell",
                 "list_mcp_resources",
@@ -183,7 +183,7 @@ async fn prompt_tools_are_consistent_across_requests() -> anyhow::Result<()> {
             ],
         ),
         (
-            "gpt-5-codex",
+            "gpt-5.1-codex",
             vec![
                 "shell",
                 "list_mcp_resources",