feat(rumqttd): configurable protocol version for WebSocket listeners

[conglp90]

Summary

WebSocket listeners are currently hardcoded to MQTT v4 (broker.rs:251), with a TODO comment:

//TODO: Add support for V5 procotol with websockets. Registered in config or on ServerSettings

This PR resolves that TODO by adding a protocol field to ServerSettings.

Changes (~27 LOC, 2 files)

src/lib.rs

• Add ProtocolVersion enum (V4, V5) with serde support and Default = V4
• Add #[serde(default)] pub protocol: ProtocolVersion to ServerSettings

src/server/broker.rs

• Import ProtocolVersion
• Match on config.protocol when spawning WS servers → Server<V4> or Server<V5>

Config example

# Default (backward compatible) — MQTT v4 over WebSocket
[ws.1]
name = "ws-v4"
listen = "0.0.0.0:8883"
next_connection_delay_ms = 1

# New — MQTT v5 over WebSocket
[ws.2]
name = "ws-v5"
listen = "0.0.0.0:8884"
next_connection_delay_ms = 1
protocol = "v5"

Backward compatibility

• protocol defaults to v4 via #[serde(default)], so existing configs work unchanged
• No changes to v4/v5 TCP listeners, network layer, or handshake logic

Testing

Tested with:

• Python paho-mqtt (MQTTv5 over WebSocket) — connect, publish, subscribe, request-response with response_topic + correlation_data
• mqtt.js browser client (protocolVersion: 5 over WS)
• All v5 features (response_topic, correlation_data) work correctly over WebSocket