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/app.rs

@@ -499,6 +499,9 @@ impl App {
                 self.chat_widget
                     .set_world_writable_warning_acknowledged(ack);
             }
+            AppEvent::UpdateRateLimitSwitchPromptHidden(hidden) => {
+                self.chat_widget.set_rate_limit_switch_prompt_hidden(hidden);
+            }
             AppEvent::PersistFullAccessWarningAcknowledged => {
                 if let Err(err) = ConfigEditsBuilder::new(&self.config.codex_home)
                     .set_hide_full_access_warning(true)
@@ -529,6 +532,21 @@ impl App {
                     ));
                 }
             }
+            AppEvent::PersistRateLimitSwitchPromptHidden => {
+                if let Err(err) = ConfigEditsBuilder::new(&self.config.codex_home)
+                    .set_hide_rate_limit_model_nudge(true)
+                    .apply()
+                    .await
+                {
+                    tracing::error!(
+                        error = %err,
+                        "failed to persist rate limit switch prompt preference"
+                    );
+                    self.chat_widget.add_error_message(format!(
+                        "Failed to save rate limit reminder preference: {err}"
+                    ));
+                }
+            }
             AppEvent::OpenApprovalsPopup => {
                 self.chat_widget.open_approvals_popup();
             }