Improve restarting behavior of the reticulate session.

[dfalbel]

Addresses #4887 by making sure the correct order of action happens:

1. Shutdown the Reticulate Python kernel
2. Restart the R session
3. Start a new reticulate session

We also added a new dialog when restarting the reticulate kernel, to make sure that the user is aware that the R kernel is also restarting, not just the reticulate kernel.

We can't restart the reticulate kernel without restarting the R session because Reticulate binds to a single Python session only once. If simply restarted Ipykernel, it would still point to the same Python session as before (eg same variables, etc).

[jmcphers]

Should this be ExitReason.Restart?

[dfalbel]

If we send a ExitReason.Restart the Jupyter Adapter will try to reinitialize it as soon as it exitted, but we actually want to wait for the new R session to be started.

positron/extensions/jupyter-adapter/src/LanguageRuntimeAdapter.ts

Lines 901 to 914 in 4a84983

	if (this._restarting) {
	this._kernel.clearSession();
	this._restarting = false;
	// Defer the start by 500ms to ensure the kernel has processed its
	// own exit before we ask it to restart. This also ensures that the
	// kernel's status events as it starts up don't overlap with the
	// status events emitted during shutdown (which can happen on the
	// Positron side due to internal buffering in the extension host
	// interface)
	setTimeout(() => {
	this._kernel.start();
	}, 500);
	}
	}

My reasonming was that this 'looks like' we are effectively switching runtimes, as we don't preserve the session RuntimeSessionObject, etc. Maybe RuntimeExitReason.Shutdown is better?

[jmcphers]

Approving since this does make things better but in my testing I noticed some intermittent issues that my be worth investigating.

First, this happened intermittently on restart:

Failed to initialize and connect to the Reticulate Python session: Cannot invoke 'reticulate_start_kernel'; no UI comm is open.

Secondly, during the restart I sometimes found that R and Python both got stuck "restarting" (according to the console toolbar) even when the restart was finished. Might be a UI bug since eventually you could use the kernels and the next operation cleared the restarting status.

[dfalbel]

I could reproduce and after some testing I figured I accidentally commited the experiment with setting this to ExitReason.Restart: #5019 (comment)

Should be working great now. I can no longer reproduce both issues.