Add basic MQTT 5.0 protocol support

[Copilot]

• Update mqtt/mqtt_protocol.h: Fix typo in MQTT_PROTOCOL_V5 comment, add AUTH packet type (v5)
• Update mqtt/mqtt_client.c: Add #include "hmath.h" for varint_decode, add helper to skip v5 properties
• Update mqtt/mqtt_client.c mqtt_client_login(): Add empty MQTT v5 properties in CONNECT packet (Variable Header properties + Will properties)
• Update mqtt/mqtt_client.c on_packet(): Skip v5 properties in CONNACK, PUBLISH, SUBACK, UNSUBACK parsing
• Update mqtt/mqtt_client.c mqtt_client_publish(): Add empty v5 properties in PUBLISH packet
• Update mqtt/mqtt_client.c mqtt_client_subscribe(): Add empty v5 properties in SUBSCRIBE packet
• Update mqtt/mqtt_client.c mqtt_client_unsubscribe(): Add empty v5 properties in UNSUBSCRIBE packet
• Update mqtt/mqtt_client.h: Add setProtocolVersion() method to C++ MqttClient class
• Build and verify changes compile
• Run code review and security checks
• Fix CONNECT comment: will_properties uses varint prefix, not fixed 2-byte

Original prompt

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This section details on the original issue you should resolve

<issue_title>请问支持mqtt5.0吗</issue_title>
<issue_description>请问支持mqtt5.0吗？</issue_description>

Comments on the Issue (you are @copilot in this section)

@ithewei 没有实现mqtt5.0的新特性，但是应该可以修改`mqtt_client_t`的`protocol_version`值，然后连接mqtt5.0服务，使用基本的发布/订阅功能。

• Fixes 请问支持mqtt5.0吗 #522

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[Copilot]

Pull request overview

Adds basic MQTT 5.0 framing compatibility to libhv’s MQTT client so it can interoperate with v5 brokers for simple CONNECT + publish/subscribe workflows by emitting empty Properties sections and skipping Properties on incoming packets.

Changes:

• Add MQTT v5 packet framing support by appending empty Properties (varint length = 0) to CONNECT (incl. Will), PUBLISH, SUBSCRIBE, and UNSUBSCRIBE outbound packets.
• Add inbound parsing support for MQTT v5 by skipping Properties on CONNACK/PUBLISH/SUBACK/UNSUBACK using a mqtt_v5_skip_properties() helper (based on varint_decode).
• Update protocol definitions (MQTT v5 no longer marked unsupported; add MQTT_TYPE_AUTH) and expose MqttClient::setProtocolVersion() in the C++ wrapper.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
mqtt/mqtt_protocol.h	Updates MQTT v5 protocol constants and adds MQTT_TYPE_AUTH.
mqtt/mqtt_client.h	Adds C++ API to set protocol_version on the underlying C client.
mqtt/mqtt_client.c	Implements minimal MQTT v5 properties framing (send empty properties; skip received properties).

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[Copilot]

The CONNECT format comment is inaccurate for MQTT v5: Will Properties are prefixed by a variable-byte integer property length (1–4 bytes), not a 2-byte length field. Consider updating the comment to avoid implying a fixed 2-byte prefix for will_properties.

Suggested change

	* [v5: 2 + will_properties] + [2 + will_topic + 2 + will_payload] +
	* [v5: varint + will_properties] + [2 + will_topic + 2 + will_payload] +

[ithewei]

@copilot apply changes based on the comments in this thread

[Copilot]

@copilot apply changes based on the comments in this thread

Fixed in 9d8b8b8 — updated the CONNECT packet format comment to use varint instead of 2 for the will_properties prefix, matching the MQTT v5 spec.