Errors
DEFER_SCOPE_SEALED
defer() registration happened after the invocation was sealed.
What failed
defer() was called after the invocation scope sealed registration (the host
already finalized the logical outcome and handed drain control to the lifetime).
Why
Sealing linearizes registration versus response commitment. Late registration would race the host response and bounds.
What still works
Callbacks already registered continue to drain under host limits. Nested registrations during an active drain wave remain allowed until nesting/count limits close them.
Fix
Register deferred work during the handler body (or nested callbacks while the drain wave is open), not after the response path has sealed the scope.