Add opt-out for rate limit model nudge (#6433)

## Summary
- add a `hide_rate_limit_model_nudge` notice flag plus config edit
plumbing so the rate limit reminder preference is persisted and
documented
- extend the chat widget prompt with a "never show again" option, and
wire new app events so selecting it hides future nudges immediately and
writes the config
- add unit coverage and refresh the snapshot for the three-option prompt

## Testing
- `just fmt`
- `just fix -p codex-tui`
- `just fix -p codex-core`
- `cargo test -p codex-tui`
- `cargo test -p codex-core` *(fails at
`exec::tests::kill_child_process_group_kills_grandchildren_on_timeout`:
grandchild process still alive)*

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_6910d7f407748321b2661fc355416994)

codex-rs/tui/src/chatwidget/tests.rs

@@ -448,6 +448,22 @@ fn rate_limit_switch_prompt_shows_once_per_session() {
     ));
 }
 
+#[test]
+fn rate_limit_switch_prompt_respects_hidden_notice() {
+    let auth = CodexAuth::create_dummy_chatgpt_auth_for_testing();
+    let (mut chat, _, _) = make_chatwidget_manual();
+    chat.config.model = "gpt-5".to_string();
+    chat.auth_manager = AuthManager::from_auth_for_testing(auth);
+    chat.config.notices.hide_rate_limit_model_nudge = Some(true);
+
+    chat.on_rate_limit_snapshot(Some(snapshot(95.0)));
+
+    assert!(matches!(
+        chat.rate_limit_switch_prompt,
+        RateLimitSwitchPromptState::Idle
+    ));
+}
+
 #[test]
 fn rate_limit_switch_prompt_defers_until_task_complete() {
     let auth = CodexAuth::create_dummy_chatgpt_auth_for_testing();