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/config/src/config_requirements.rs

@@ -2513,17 +2513,6 @@ allowed_approvals_reviewers = ["user"]
                 .can_set(&AskForApproval::UnlessTrusted)
                 .is_ok()
         );
-        assert_eq!(
-            requirements
-                .approval_policy
-                .can_set(&AskForApproval::OnFailure),
-            Err(ConstraintError::InvalidValue {
-                field_name: "approval_policy",
-                candidate: "OnFailure".into(),
-                allowed: "[UnlessTrusted, OnRequest]".into(),
-                requirement_source: RequirementSource::Unknown,
-            })
-        );
         assert!(
             requirements
                 .approval_policy

codex-rs/config/src/state_tests.rs

@@ -52,7 +52,7 @@ fn active_user_layer_is_highest_precedence_user_layer() {
         toml::from_str(
             r#"
 model = "base"
-approval_policy = "on-failure"
+approval_policy = "on-request"
 "#,
         )
         .expect("base config"),
@@ -86,7 +86,7 @@ approval_policy = "on-failure"
             .expect("merged user config")
             .get("approval_policy")
             .and_then(toml::Value::as_str),
-        Some("on-failure")
+        Some("on-request")
     );
 }
 
@@ -107,7 +107,7 @@ fn with_user_config_updates_matching_user_layer_without_replacing_active_profile
             file: profile_file.clone(),
             profile: Some("work".to_string()),
         },
-        toml::from_str(r#"approval_policy = "on-failure""#).expect("profile config"),
+        toml::from_str(r#"approval_policy = "on-request""#).expect("profile config"),
     );
     let stack = ConfigLayerStack::new(
         vec![base_layer, profile_layer],
@@ -136,6 +136,6 @@ fn with_user_config_updates_matching_user_layer_without_replacing_active_profile
             .expect("merged user config")
             .get("approval_policy")
             .and_then(toml::Value::as_str),
-        Some("on-failure")
+        Some("on-request")
     );
 }