chore(core) Deprecate approval_policy: on-failure (#11631)

## Summary
In an effort to start simplifying our sandbox setup, we're announcing
this approval_policy as deprecated. In general, it performs worse than
`on-request`, and we're focusing on making fewer sandbox configurations
perform much better.

## Testing
- [x] Tested locally
- [x] Existing tests pass

codex-rs/core/src/codex.rs

@@ -1099,6 +1099,14 @@ impl Session {
             });
         }
         maybe_push_unstable_features_warning(&config, &mut post_session_configured_events);
+        if config.approval_policy.value() == AskForApproval::OnFailure {
+            post_session_configured_events.push(Event {
+                id: "".to_owned(),
+                msg: EventMsg::Warning(WarningEvent {
+                    message: "`on-failure` approval policy is deprecated and will be removed in a future release. Use `on-request` for interactive approvals or `never` for non-interactive runs.".to_string(),
+                }),
+            });
+        }
 
         let auth = auth.as_ref();
         let auth_mode = auth.map(CodexAuth::auth_mode).map(TelemetryAuthMode::from);