Clear non-empty prompts with ctrl + c (#3285)

This updates the ctrl + c behavior to clear the current prompt if there
is text and you press ctrl + c.

I also updated the ctrl + c hint text to show `^c to interrupt` instead
of `^c to quit` if there is an active conversation.

Two things I don't love:
1. You can currently interrupt a conversation with escape or ctrl + c
(not related to this PR and maybe fine)
2. The bottom row hint text always says `^c to quit` but this PR doesn't
really make that worse.




https://github.com/user-attachments/assets/6eddadec-0d84-4fa7-abcb-d6f5a04e5748


Fixes https://github.com/openai/codex/issues/3126

codex-rs/tui/src/bottom_pane/bottom_pane_view.rs

@@ -19,7 +19,7 @@ pub(crate) trait BottomPaneView {
 
     /// Handle Ctrl-C while this view is active.
     fn on_ctrl_c(&mut self, _pane: &mut BottomPane) -> CancellationEvent {
-        CancellationEvent::Ignored
+        CancellationEvent::NotHandled
     }
 
     /// Return the desired height of the view.