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/protocol/src/protocol.rs

@@ -870,15 +870,8 @@ pub enum AskForApproval {
     #[strum(serialize = "untrusted")]
     UnlessTrusted,
 
-    /// DEPRECATED: *All* commands are auto‑approved, but they are expected to
-    /// run inside a sandbox where network access is disabled and writes are
-    /// confined to a specific set of paths. If the command fails, it will be
-    /// escalated to the user to approve execution without a sandbox.
-    /// Prefer `OnRequest` for interactive runs or `Never` for non-interactive
-    /// runs.
-    OnFailure,
-
     /// The model decides when to ask the user for approval.
+    #[serde(alias = "on-failure")]
     #[default]
     OnRequest,
 
@@ -5439,6 +5432,20 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn turn_context_item_deserializes_legacy_on_failure_as_on_request() -> Result<()> {
+        let item: TurnContextItem = serde_json::from_value(json!({
+            "cwd": test_path_buf("/tmp"),
+            "approval_policy": "on-failure",
+            "sandbox_policy": { "type": "danger-full-access" },
+            "model": "gpt-5",
+            "summary": "auto",
+        }))?;
+
+        assert_eq!(item.approval_policy, AskForApproval::OnRequest);
+        Ok(())
+    }
+
     #[test]
     fn multi_agent_version_uses_newest_present_session_meta_value() -> Result<()> {
         let thread_id = ThreadId::from_string("67e55044-10b1-426f-9247-bb680e5fe0c8")?;