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/common.rs

@@ -2604,7 +2604,7 @@ mod tests {
                         "/tmp/AGENTS.md",
                     )),
                 ],
-                approval_policy: v2::AskForApproval::OnFailure,
+                approval_policy: v2::AskForApproval::OnRequest,
                 approvals_reviewer: v2::ApprovalsReviewer::User,
                 sandbox: v2::SandboxPolicy::DangerFullAccess,
                 active_permission_profile: None,
@@ -2651,7 +2651,7 @@ mod tests {
                     "cwd": absolute_path_string("tmp"),
                     "runtimeWorkspaceRoots": [],
                     "instructionSources": [absolute_path_string("tmp/AGENTS.md")],
-                    "approvalPolicy": "on-failure",
+                    "approvalPolicy": "on-request",
                     "approvalsReviewer": "user",
                     "sandbox": {
                         "type": "dangerFullAccess"