96db73d1c6
Add DeliverRetainedOnSubscribe to MqttRetainedStore, which iterates GetMatchingRetained and fires a callback with (topic, payload, qos, retain=true) for each match. Add 11 unit tests covering exact-match, '+' single-level, '#' multi-level, no-match, cross-level rejection, callback arguments, return count, and empty-store behaviour.