Add synchronous in-memory dedup at EventDispatcher handler level using
message_id as key with 5-minute TTL and 2000-entry cap.
This catches duplicate events immediately when they arrive from the Lark
SDK — before the inbound debouncer or processing queue — preventing the
race condition where two concurrent dispatches enter the pipeline before
either records the messageId in the downstream dedup layer.
Fixes the root cause reported in #42687.
a8ae682bda