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/codex-mcp/src/elicitation.rs

@@ -247,7 +247,6 @@ impl ElicitationRequestManager {
 pub(crate) fn elicitation_is_rejected_by_policy(approval_policy: AskForApproval) -> bool {
     match approval_policy {
         AskForApproval::Never => true,
-        AskForApproval::OnFailure => false,
         AskForApproval::OnRequest => false,
         AskForApproval::UnlessTrusted => false,
         AskForApproval::Granular(granular_config) => !granular_config.allows_mcp_elicitations(),