Fix MQTT QoS 2 Subscriber Handling (PUBREC/PUBREL/PUBCOMP) and Add Tests

[MarcAntoineCRUE]

Description:

This merge request fixes critical issues in the MQTT QoS 2 message flow for subscribers in the PubSubClient library, and adds a dedicated test to ensure correct behavior.

Summary of changes:

• Correctly respond with PUBREC (not PUBACK) when receiving a QoS 2 PUBLISH as a subscriber.
• Add missing handler for PUBREL packets: the client now responds with PUBCOMP, completing the QoS 2 handshake as per the MQTT 3.1.1 specification.
• Update comments for clarity regarding the QoS 2 handshake steps.
• Add a new test (test_receive_qos2) to verify the full QoS 2 subscriber flow: PUBLISH → PUBREC → PUBREL → PUBCOMP, and ensure the callback is only called once.
• All existing and new tests pass, with no regressions.

Motivation:

Previously, the library responded with PUBACK to all QoS > 0 PUBLISH packets, breaking the QoS 2 protocol. The lack of a PUBREL handler meant the handshake was never completed, potentially causing message loss or duplicate delivery. This patch brings the implementation in line with the MQTT specification and ensures robust, standards-compliant behavior.

Impact:

• Fixes MQTT QoS 2 reliability for subscribers.
• No breaking changes for existing users.
• Improves test coverage and protocol compliance.

[hmueller01]

@MarcAntoineCRUE Thank you very much for your contribution!
This is a nice catch! Obviously no one checked that before ...

Could you please do not commit a new test (tests/src/receive_spec copy.cpp) but instead update the current receive_spec.cpp?

[MarcAntoineCRUE]

Yes I just mentioned I added a new commit, this is why I changed it to draft, as I need to update that
I will change status as ready when I will have corrected that

[MarcAntoineCRUE]

It is now good for review

[hmueller01]

LGTM

[hmueller01]

Can you please add short change note in CHANGELOG.md ## [Unreleased] section. Thanks a lot!

[github-actions]

Memory usage change @ a5710c2

Board	flash	%	RAM for global variables	%
arduino:avr:uno	🔺 +28 - +36	+0.09 - +0.11	0 - 0	0.0 - 0.0
arduino:megaavr:uno2018	🔺 +28 - +36	+0.06 - +0.07	0 - 0	0.0 - 0.0
arduino:samd:mkr1000	🔺 +24 - +24	+0.01 - +0.01	0 - 0	0.0 - 0.0
esp32:esp32:esp32	🔺 +28 - +32	0.0 - 0.0	0 - 0	0.0 - 0.0
esp8266:esp8266:generic	N/A	N/A	N/A	N/A

Click for full report table

Board	examples/mqtt_auth flash	%	examples/mqtt_auth RAM for global variables	%	examples/mqtt_basic flash	%	examples/mqtt_basic RAM for global variables	%	examples/mqtt_progmem flash	%	examples/mqtt_progmem RAM for global variables	%	examples/mqtt_publish_in_callback flash	%	examples/mqtt_publish_in_callback RAM for global variables	%	examples/mqtt_reconnect_nonblocking flash	%	examples/mqtt_reconnect_nonblocking RAM for global variables	%	examples/mqtt_stream flash	%	examples/mqtt_stream RAM for global variables	%	examples/mqtt_esp flash	%	examples/mqtt_esp RAM for global variables	%	examples/mqtt_esp_large_message flash	%	examples/mqtt_esp_large_message RAM for global variables	%
arduino:avr:uno	36	0.11	0	0.0	34	0.11	0	0.0	36	0.11	0	0.0	36	0.11	0	0.0	28	0.09	0	0.0	34	0.11	0	0.0
arduino:megaavr:uno2018	36	0.07	0	0.0	36	0.07	0	0.0	36	0.07	0	0.0	36	0.07	0	0.0	28	0.06	0	0.0	36	0.07	0	0.0
arduino:samd:mkr1000	24	0.01	0	0.0	24	0.01	0	0.0	24	0.01	0	0.0	24	0.01	0	0.0	24	0.01	0	0.0	24	0.01	0	0.0
esp32:esp32:esp32	32	0.0	0	0.0	32	0.0	0	0.0	32	0.0	0	0.0	32	0.0	0	0.0	32	0.0	0	0.0					28	0.0	0	0.0	28	0.0	0	0.0
esp8266:esp8266:generic	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A

Click for full report CSV

Board,examples/mqtt_auth<br>flash,%,examples/mqtt_auth<br>RAM for global variables,%,examples/mqtt_basic<br>flash,%,examples/mqtt_basic<br>RAM for global variables,%,examples/mqtt_progmem<br>flash,%,examples/mqtt_progmem<br>RAM for global variables,%,examples/mqtt_publish_in_callback<br>flash,%,examples/mqtt_publish_in_callback<br>RAM for global variables,%,examples/mqtt_reconnect_nonblocking<br>flash,%,examples/mqtt_reconnect_nonblocking<br>RAM for global variables,%,examples/mqtt_stream<br>flash,%,examples/mqtt_stream<br>RAM for global variables,%,examples/mqtt_esp<br>flash,%,examples/mqtt_esp<br>RAM for global variables,%,examples/mqtt_esp_large_message<br>flash,%,examples/mqtt_esp_large_message<br>RAM for global variables,%
arduino:avr:uno,36,0.11,0,0.0,34,0.11,0,0.0,36,0.11,0,0.0,36,0.11,0,0.0,28,0.09,0,0.0,34,0.11,0,0.0
arduino:megaavr:uno2018,36,0.07,0,0.0,36,0.07,0,0.0,36,0.07,0,0.0,36,0.07,0,0.0,28,0.06,0,0.0,36,0.07,0,0.0
arduino:samd:mkr1000,24,0.01,0,0.0,24,0.01,0,0.0,24,0.01,0,0.0,24,0.01,0,0.0,24,0.01,0,0.0,24,0.01,0,0.0
esp32:esp32:esp32,32,0.0,0,0.0,32,0.0,0,0.0,32,0.0,0,0.0,32,0.0,0,0.0,32,0.0,0,0.0,,,,,28,0.0,0,0.0,28,0.0,0,0.0
esp8266:esp8266:generic,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A

[MarcAntoineCRUE]

Ok changelog is done, up to you for merging