Fix MQTT protocol compliance: QoS 2 flow, SUBACK, add DISCONNECT handler

[Copilot]

Addresses MQTT 3.1.1 protocol compliance gaps and missing command handler.

Bug Fixes

• QoS 2 flow: PUBREC now responds with PUBREL (0x62), PUBREL responds with PUBCOMP (0x70). Previously both echoed their own packet type back.
• SUBACK response: Returns granted QoS per topic filter instead of only the first topic's QoS.

New Features

• DISCONNECT handler: Closes session gracefully on client disconnect. Topic cleanup handled by existing TopicMiddleware.UnRegisterSession.

Tests

Added 22 tests (48 total) covering:

• All command handlers including edge cases (empty topic, no subscribers, multiple subscribers)
• Packet encoding/decoding via MQTTPacketDecoder
• Complete QoS 2 flow sequence
• TopicMiddleware session cleanup

// QoS 2 flow now correct:
// Client PUBREC → Server PUBREL (0x62)
// Client PUBREL → Server PUBCOMP (0x70)

// SUBACK now returns QoS per topic:
// Subscribe to 2 topics with QoS 0 and 1
// Response: [0x90, 0x04, packetId_hi, packetId_lo, 0x00, 0x01]

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.