Implement key MQTT 5 features not available in MQTT 3.1.1: user properties, reason codes, and shared subscriptions

Steps

1. Upgrade broker and clients to MQTT 5 support (e.g., EMQX, HiveMQ, Mosquitto 2.x); verify version negotiation succeeds in the CONNACK — if the broker downgrades to 3.1.1, MQTT 5 features will be silently unavailable
2. Use User Properties in PUBLISH packets to carry metadata (device type, firmware version, geographic region) as key-value pairs without embedding them in the payload; brokers forward user properties transparently to subscribers
3. Read Reason Codes on CONNACK, SUBACK, PUBACK, and DISCONNECT to get specific failure reasons (e.g., reason code 0x97 = Quota Exceeded, 0x87 = Not Authorized) instead of the opaque failure booleans of MQTT 3.1.1
4. Use Shared Subscriptions ($share/<group>/<topic>) to distribute messages across multiple consumer instances for horizontal scaling; the broker delivers each message to exactly one member of the share group in round-robin or random fashion
5. Set Message Expiry Interval on QoS 1/2 messages to discard stale commands; set Session Expiry Interval in CONNECT to control how long the broker retains session state after disconnect — use 0 for clean sessions or a nonzero value for persistent sessions
6. Use the Will Delay Interval in MQTT 5 LWT to defer publishing the will message; set it longer than the expected reconnection window so transient network glitches do not trigger false offline alerts