6.2.1 added the missing handlers.set call but EffectHandlers.set is
typed as `entityId: string`; passing `string | undefined` blew the
DTS build. Coerce to '' so the type contract holds — the existing
server set handler (and the runtime UI wrapper) both treat '' as
"no entity id" and fall back to the in-memory mirror path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>