Add optional IStreamStore backing to MqttSessionStore and MqttRetainedStore,
enabling session and retained message state to survive process restarts via
JetStream persistence. Includes ConnectAsync/SaveSessionAsync for session
lifecycle, SetRetainedAsync/GetRetainedAsync with cleared-topic tombstone
tracking, and 4 new parity tests covering persist/restart/clear semantics.
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.
