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/session/mod.rs

@@ -2246,8 +2246,7 @@ impl Session {
                     strict_auto_review: false,
                 });
             }
-            AskForApproval::OnFailure
-            | AskForApproval::OnRequest
+            AskForApproval::OnRequest
             | AskForApproval::UnlessTrusted
             | AskForApproval::Granular(_) => {}
         }