Fixes for various fenrir issues

[embhorn]

Fixes for:

• Doc f487 - props lock
• Fix f793 - return on MqttPacket_Write failure
• Fix f795 - SN non-blocking read issue
• Fix f796 - SN NPD
• Fix f806 - MqttDecode_Auth reason code issue
• Fix f807-810 SN Decode check buf
• Fix f798 - SN_Encode_Publish topic_name pointer access
• Fix f799 - SN_Decode_SubscribeAck wrong error code
• Fix f800 - SN_Decode_Publish length calc

[Copilot]

Pull request overview

This PR addresses a set of MQTT v5 and MQTT-SN correctness/stability issues (buffer validation, non-blocking read behavior, error propagation, and property init/shutdown documentation).

Changes:

• Add/clarify thread-safety documentation for MQTT v5 property init/shutdown APIs.
• Harden multiple MQTT-SN decode paths with additional bounds checks and fix decode/encode edge cases (topic id access, publish length calc, subscribe-ack error code).
• Fix MQTT v5 AUTH decode logic and ensure early return on write failures in MQTT-SN client flows.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
wolfmqtt/mqtt_packet.h	Documents single-thread init/shutdown requirements for MQTT v5 property APIs.
src/mqtt_sn_packet.c	Adds bounds checks to MQTT-SN decoders and fixes publish encoding/length and packet read behavior.
src/mqtt_sn_client.c	Returns immediately on packet write failure in several MQTT-SN client operations.
src/mqtt_packet.c	Fixes MQTT v5 AUTH decoding behavior and documents property init/shutdown semantics.

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[Copilot]

Pull request overview

This PR applies a set of targeted fixes across MQTT-SN and MQTT v5 AUTH/property handling, addressing multiple fenrir-reported issues and improving robustness around packet decoding and non-blocking I/O paths.

Changes:

• Add thread-usage documentation for MQTT v5 property init/shutdown APIs.
• Harden MQTT-SN decoders (length checks, correct error codes, safer topic-id access, publish length calculation).
• Adjust MQTT-SN packet read logic and MQTT v5 AUTH decode behavior (including Remaining Length = 0 semantics).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
wolfmqtt/mqtt_packet.h	Documents required single-thread init/shutdown semantics for MQTT v5 property helpers.
src/mqtt_sn_packet.c	Adds MQTT-SN decode bounds checks and refactors publish decode and packet read logic.
src/mqtt_sn_client.c	Ensures early returns on MqttPacket_Write failure in selected SN client operations.
src/mqtt_packet.c	Fixes MQTT v5 AUTH decode semantics and adds property init/shutdown comments in implementation.

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