The previous dedupe map rotation logic completely cleared the map when it reached max size, causing an 'amnesia cliff' where immediately arriving duplicates of just-forgotten messages would be processed.
This change replaces that with a MessageDeduplicator struct that uses a circular queue (ring buffer) to track insertions. When the limit is reached, it only evicts the absolute oldest message from the map, completely resolving the cliff issue.
This also cleans up the WeCom Bot and App webhook handlers by encapsulating the mutex and map state.
When the dedupe map rotates, the previous logic entirely cleared the map, meaning the message that triggered the rotation was immediately forgotten and could be duplicated immediately.
This change seeds the new map with the current message to prevent that. Also adds a defensive nil check.
Match rotation semantics to prior behavior by fully resetting the dedupe map
once the size limit is exceeded, and add focused tests for duplicate detection
and boundary rotation behavior.
