Add FlapperState class and per-client exponential backoff tracking to
MqttSessionStore. New TrackConnectDisconnect(string) overload returns
FlapperState with backoff level and expiry; IsFlapper, GetBackoffMs,
ClearFlapperState, and CheckAndClearStableClients give callers full
visibility and cleanup control. Legacy two-arg overload preserved for
backward compatibility. Ten unit tests cover counting, threshold,
exponential growth, 60s cap, window reset, and stable-client sweep.
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.
Implements MqttFlowController with per-subscription SemaphoreSlim-based
slot tracking for QoS 1/2 messages. Replaces the previous catch-and-swallow
pattern with an explicit CurrentCount guard on Release. 10 unit tests added.
Adds WillMessage class, SetWill/ClearWill/GetWill methods to MqttSessionStore,
PublishWillMessage that dispatches via OnPublish delegate (or tracks as delayed
when DelayIntervalSeconds > 0), and 10 unit tests covering all will message behaviors.
Add MqttSessionStore with save/load/delete/list operations, flapper
detection (backoff on rapid reconnects), and TimeProvider-based testing.
Add MqttRetainedStore for per-topic retained messages with MQTT wildcard
matching (+/# filters). Add MqttQos2StateMachine tracking the full
PUBREC/PUBREL/PUBCOMP flow with duplicate rejection and timeout detection.
19 new tests: 9 session persistence, 10 QoS/retained message tests.
