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/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -6864,7 +6864,6 @@
           {
             "enum": [
               "untrusted",
-              "on-failure",
               "on-request",
               "never"
             ],