Improve runtime session service stability (#5380)

The aim of this PR is to improve the stability of the runtime session
service, with a slight focus on notebooks.

This is another step toward
#2671.

I've also added an extensive suite of unit tests for the runtime session
service, for two reasons:

1. Some of the issues we're seeing with notebooks are intermittent and
therefore tricky to reproduce without tests.
2. I wanted to ensure that I didn't break any existing behavior with
these changes as well as more planned changes to add notebook-runtime
methods to the extension API.

extensions/positron-notebook-controllers/src/notebookController.ts

@@ -331,10 +331,19 @@ function executeCode(
 					}
 				}
 			}).catch(error => {
+				// Stop listening for replies.
 				handler.dispose();
+
+				// Reject the outer execution promise since we've encountered an error.
 				reject(error);
+
+				// Rethrow the error to stop any replies that are chained to this promise.
 				throw error;
 			});
+
+			// Avoid unhandled rejections being logged to the console.
+			// The actual error-handling is in the catch block above.
+			currentMessagePromise.catch(() => { });
 		});
 
 		// Execute the cell.

src/vs/workbench/contrib/positronPreview/browser/positronPreview.contribution.ts

@@ -20,12 +20,11 @@ import { ViewContainer, IViewContainersRegistry, ViewContainerLocation, Extensio
 import { registerAction2 } from 'vs/platform/actions/common/actions';
 import { PositronOpenUrlInViewerAction } from 'vs/workbench/contrib/positronPreview/browser/positronPreviewActions';
 import { IConfigurationRegistry, Extensions as ConfigurationExtensions, ConfigurationScope, } from 'vs/platform/configuration/common/configurationRegistry';
+import { POSITRON_PREVIEW_PLOTS_IN_VIEWER } from 'vs/workbench/services/languageRuntime/common/languageRuntimeUiClient';
 
 // The Positron preview view icon.
 const positronPreviewViewIcon = registerIcon('positron-preview-view-icon', Codicon.positronPreviewView, nls.localize('positronPreviewViewIcon', 'View icon of the Positron preview view.'));
 
-export const POSITRON_PREVIEW_PLOTS_IN_VIEWER = 'positron.viewer.interactivePlotsInViewer';
-
 // Register the Positron preview container.
 const VIEW_CONTAINER: ViewContainer = Registry.as<IViewContainersRegistry>(ViewContainerExtensions.ViewContainersRegistry).registerViewContainer({
 	id: POSITRON_PREVIEW_VIEW_ID,

src/vs/workbench/services/languageRuntime/common/languageRuntimeUiClient.ts

@@ -13,8 +13,8 @@ import { URI } from 'vs/base/common/uri';
 import { ICommandService } from 'vs/platform/commands/common/commands';
 import { ILogService } from 'vs/platform/log/common/log';
 import { IConfigurationService } from 'vs/platform/configuration/common/configuration';
-import { POSITRON_PREVIEW_PLOTS_IN_VIEWER } from 'vs/workbench/contrib/positronPreview/browser/positronPreview.contribution';
 
+export const POSITRON_PREVIEW_PLOTS_IN_VIEWER = 'positron.viewer.interactivePlotsInViewer';
 
 /**
  * The types of messages that can be sent to the backend.