Add ConnectUnixAsync to DBusTransport

[jmacato]

Summary

• Adds async overloads of ConnectUnix to DBusTransport that use socket.ConnectAsync() with CancellationToken support
• Enables non-blocking Unix domain socket connects for peer-to-peer D-Bus connections
• Used by Babylon's DBusAutomation client to allow proper cancellation and retry on connection refused

Test plan

• Verify existing ConnectUnix (sync) tests still pass
• Verify ConnectUnixAsync connects to a listening Unix socket
• Verify cancellation token aborts the connect and disposes the socket

[Copilot]

Pull request overview

This PR adds async overloads of ConnectUnix to the DBusTransport class to enable non-blocking Unix domain socket connections for peer-to-peer D-Bus communication with proper cancellation token support.

Changes:

• Adds ConnectUnixAsync(string socketPath, CancellationToken) overload for basic async connection
• Adds ConnectUnixAsync(string socketPath, IDBusDiagnostics?, CancellationToken) overload with diagnostics support
• Implements proper exception handling with socket disposal on connection failure
• Follows the same pattern as the existing synchronous ConnectUnix methods

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