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/core/src/config/config_loader_tests.rs

@@ -640,7 +640,7 @@ async fn selected_user_config_file_layers_over_base_user_config() {
         tmp.path().join(CONFIG_TOML_FILE),
         r#"
 model = "gpt-main"
-approval_policy = "on-failure"
+approval_policy = "on-request"
 "#,
     )
     .expect("write default user config");
@@ -697,7 +697,7 @@ approval_policy = "on-failure"
             .effective_config()
             .get("approval_policy")
             .and_then(TomlValue::as_str),
-        Some("on-failure")
+        Some("on-request")
     );
 }
 
@@ -1033,12 +1033,6 @@ personality = true
     config_requirements
         .approval_policy
         .can_set(&AskForApproval::Never)?;
-    assert!(
-        config_requirements
-            .approval_policy
-            .can_set(&AskForApproval::OnFailure)
-            .is_err()
-    );
     assert_eq!(
         config_requirements.web_search_mode.value(),
         WebSearchMode::Cached

codex-rs/core/src/config/config_tests.rs

@@ -8272,7 +8272,7 @@ model_provider = "openai-custom"
 [profiles.zdr]
 model = "o3"
 model_provider = "openai"
-approval_policy = "on-failure"
+approval_policy = "on-request"
 
 [profiles.zdr.analytics]
 enabled = false
@@ -8280,7 +8280,7 @@ enabled = false
 [profiles.gpt5]
 model = "gpt-5.4"
 model_provider = "openai"
-approval_policy = "on-failure"
+approval_policy = "on-request"
 model_reasoning_effort = "high"
 model_reasoning_summary = "detailed"
 model_verbosity = "high"