* fix: confirm response based on promise

When using the new `workbench.action.chat.open.blockOnResponse`
feature, if the VSC instance was in auto-approve mode, it was observed
that the command was resolving before the tool finished invocation.

This resulted in a race betwee sync and async code:

```
		if (!this._confirmationMessages) {
			// No confirmation needed
			this._isConfirmed = { type: ToolConfirmKind.ConfirmationNotNeeded };
			this._confirmDeferred.complete(this._isConfirmed);
		}
```

and then the check for confirmation in `isConfirmed`.

* remove _isConfirmed in favor of always the promise