Authentication provider registration of VSCode based extensions not working

[rijad992]

Bug Description:

After implementation of #16252 Vscode based extensions that register authentication provider are unable to do so.
On theia startup error is thrown from this line authenticationProvider.onDidChangeSession is not a function. Seems like at this point authenticationProvider is instance of AuthenticationProviderImpl instead of registered custom provider.

Accounts button in bottom left never gets shown:

Steps to Reproduce:

1. Install vscode based extension that registers authentication provider, for example https://github.com/mikeeeyy04/GitShift/releases
   This one registers auth provider. Download vsix and install manually. If you got other example of extension that
   registers auth provider feel free to use it.
2. Start theia, you will notice in logs onDidChangeSession is not a function error and no accounts button will appear.
3. If you just comment out line it will work.

Additional Information

• Operating System: Windows 11
• Theia Version: 1.66.2

[ndoschek]

Thanks for the detailed report @rijad992, and for digging into the root cause already!
Would you be interested in proposing a fix, we'd appreciate a contribution here, and we’re happy to support you along the way if needed.