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_event.rs

@@ -104,6 +104,9 @@ pub(crate) enum AppEvent {
     #[cfg_attr(not(target_os = "windows"), allow(dead_code))]
     UpdateWorldWritableWarningAcknowledged(bool),
 
+    /// Update whether the rate limit switch prompt has been acknowledged for the session.
+    UpdateRateLimitSwitchPromptHidden(bool),
+
     /// Persist the acknowledgement flag for the full access warning prompt.
     PersistFullAccessWarningAcknowledged,
 
@@ -111,6 +114,9 @@ pub(crate) enum AppEvent {
     #[cfg_attr(not(target_os = "windows"), allow(dead_code))]
     PersistWorldWritableWarningAcknowledged,
 
+    /// Persist the acknowledgement flag for the rate limit switch prompt.
+    PersistRateLimitSwitchPromptHidden,
+
     /// Skip the next world-writable scan (one-shot) after a user-confirmed continue.
     #[cfg_attr(not(target_os = "windows"), allow(dead_code))]
     SkipNextWorldWritableScan,