CmdPal: Safely handle invoking an extension that died #51443

This is an issue I noticed while debugging the dock with a dev build. If I quit out of the dev build, then try to click on a dock item on my stable build. If that item was from an extension, I would no longer be able to run any commands in command palette at all.

Turns out the issue is much bigger than just dock items. It seems if we tried to perform the command for any item from an extension that has died, we would throw an exception outside of the try/catch in ShellViewModel::PerformCommand. That would cause us to never release the _handleInvokeTask, and we'd never be able to start invoking another command.

Looks like the call to command.Unsave.Name got moved from inside the try/catch to outside in #43905. Whoops.

While I was here, I thought it best that we remove this footgun. PerformCommandMessage now carries around a CommandViewModel instead of just an ExtensionObject<ICommand>, so that consumers can use the cached name & Id rather than begging to use .Unsafe on it.

Closes: probably nothing, maybe something though?