Properly close extension

[davidegiacometti]

Properly close the extension when CmdPal is closed (porting of microsoft/PowerToys#39209)
Note that microsoft/PowerToys#39589 is needed in order to properly close extensions when CmdPal is closed from PowerToys system tray.

Validation steps:

• Launch CmdPal
• Verify that GitHubExtension process is running
• Invoke CmdPal
• Press Alt+F4
• Verify that GitHubExtension process is closed

[Copilot]

Pull Request Overview

This PR ensures that the GitHubExtension properly shuts down when CmdPal is closed.

• Added explicit server shutdown and disposal after waiting for the extension to be disposed.
• Ensured that the server is stopped and disposed immediately to prevent lingering processes.

Comments suppressed due to low confidence (1)

GitHubExtension/Program.cs:175

• [nitpick] Verify that using UnsafeDispose is appropriate in this context; if a safer disposal method exists, consider using it to avoid bypassing potential safety checks during shutdown.

server.UnsafeDispose();