Extend PowerShell module with async message handlers and MQTT 5 features

[Copilot]

Implements the async message handler proposal from #2215 and exposes additional MQTTnet features as PowerShell cmdlets.

Core Implementation

RegisterMqttMessageHandler - Async handler with proper runspace management:

• Executes ScriptBlock in captured runspace via Task.Run
• Passes ($topic, $payload, $message) to handler
• Error output to host stream without blocking event thread

Register-MqttMessageHandler -Session $session -Action {
    param($topic, $payload, $message)
    Write-Host "Received on $topic: $payload (QoS: $($message.QoS))"
}

Connection Enhancements

Connect-MqttSession - Added transport and protocol options:

• WebSocket: -WebSocketUri "ws://broker:8000/mqtt"
• TLS: -UseTls -AllowUntrustedCertificates
• LWT: -WillTopic, -WillPayload, -WillQoS, -WillRetain
• Tuning: -KeepAlivePeriod, -Timeout

Publishing Enhancements

Publish-MqttMessage - MQTT 5 user properties:

$props = @{ "source" = "PowerShell"; "version" = "1.0" }
Publish-MqttMessage -Session $session -Topic "data" -Payload "test" -UserProperties $props

New Cmdlets

• Get-MqttSessionStatus - Connection state
• Test-MqttConnection - Broker ping with timeout
• Unregister-MqttMessageHandler - Clear all handlers
• Show-MqttExamples - Quick reference

Documentation

• README.md with examples and parameter documentation
• Test.ps1 with comprehensive usage scenarios
• SmokeTest.ps1 for validation

Total: 13 cmdlets (7 new, 2 enhanced, 4 existing)

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