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/src/protocol/v2/shared.rs

@@ -163,7 +163,6 @@ pub enum AskForApproval {
     #[serde(rename = "untrusted")]
     #[ts(rename = "untrusted")]
     UnlessTrusted,
-    OnFailure,
     OnRequest,
     #[experimental("askForApproval.granular")]
     Granular {
@@ -182,7 +181,6 @@ impl AskForApproval {
     pub fn to_core(self) -> CoreAskForApproval {
         match self {
             AskForApproval::UnlessTrusted => CoreAskForApproval::UnlessTrusted,
-            AskForApproval::OnFailure => CoreAskForApproval::OnFailure,
             AskForApproval::OnRequest => CoreAskForApproval::OnRequest,
             AskForApproval::Granular {
                 sandbox_approval,
@@ -206,7 +204,6 @@ impl From<CoreAskForApproval> for AskForApproval {
     fn from(value: CoreAskForApproval) -> Self {
         match value {
             CoreAskForApproval::UnlessTrusted => AskForApproval::UnlessTrusted,
-            CoreAskForApproval::OnFailure => AskForApproval::OnFailure,
             CoreAskForApproval::OnRequest => AskForApproval::OnRequest,
             CoreAskForApproval::Granular(granular_config) => AskForApproval::Granular {
                 sandbox_approval: granular_config.sandbox_approval,