Disconnect when receiving an unsuccessful CONNACK

[smaeul]

MQTT v3.1.1 requires: "If a server sends a CONNACK packet containing a non-zero return code it MUST then close the Network Connection [MQTT-3.2.2-5]."

MQTT v5.0 requires: "If a Server sends a CONNACK packet containing a Reason code of 128 or greater it MUST then close the Network Connection [MQTT-3.2.2-7]."

Since the network connection is guaranteed to be closed from the server side, the library can detect this and request to close the socket on the client side.

The user could handle this by separately parsing the NotifyPacketReceived(Connack) event, but this seems like a reasonable convenience to provide for clients that don't care about the reason code, if you agree. This ordering was simplest to implement, but may be the RequestClose event should come after the NotifyPacketReceived?

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.88%. Comparing base (609c3a3) to head (fc98b6f).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mqtt/connection/core.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
- Coverage   90.90%   90.88%   -0.03%     
==========================================
  Files          55       55              
  Lines        7456     7458       +2     
==========================================
  Hits         6778     6778              
- Misses        678      680       +2     

Files with missing lines	Coverage Δ
src/mqtt/connection/core.rs	86.26% <0.00%> (-0.11%)	⬇️

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[redboltz]

According to the MQTT specification, when a CONNACK packet with an error and/or a DISCONNECT packet is sent, the sender must close the connection. This allows the underlying layers to shut down gracefully.
If the receiver also initiates a close, it may interfere with this graceful shutdown process. Therefore, mqtt-protocol-core does not close the connection from the receiver side.

If the underlying layer does not detect that the connection has been closed within a certain period after receiving a CONNACK-with-error or DISCONNECT packet, the user may choose to close the connection manually.

[smaeul]

OK, I can handle this in my event loop.