chore(core) rm AskForApproval::OnFailure (#28418)

## Summary
Deletes the OnFailure variant of the `AskForApproval` enum. This option
has been deprecated since #11631.

## Testing
- [x] Tests pass

codex-rs/core/src/hook_runtime.rs

@@ -738,10 +738,9 @@ fn hook_run_metric_tags(run: &HookRunSummary) -> [(&'static str, &'static str);
 fn hook_permission_mode(turn_context: &TurnContext) -> String {
     match turn_context.approval_policy.value() {
         AskForApproval::Never => "bypassPermissions",
-        AskForApproval::UnlessTrusted
-        | AskForApproval::OnFailure
-        | AskForApproval::OnRequest
-        | AskForApproval::Granular(_) => "default",
+        AskForApproval::UnlessTrusted | AskForApproval::OnRequest | AskForApproval::Granular(_) => {
+            "default"
+        }
     }
     .to_string()
 }