Move notebook runtime URI fixing to the main-thread. (#4017)

Addresses #4016. 

Move the reviving of a URI to `ExtHostLanguageRuntime.$createLanguageRuntimeSession()` when starting a runtime.
Fixes problem where sometimes the URI would get silently blown away by
bad to-string conversion and then the runtime would not know where it
was supposed to be running.

Previously I added a URI "revival" step on the `JupyterKernel` code to deal with the fact that positron notebooks sometimes provide non-fully-formed URIs. Turns out this didn't actually work and just resulted in undefined variables due to the serialized URI-like object not having a`.toString()` method implemented. This caused the notebook to start from the vaunted `[Object object]` path. 
I thought it worked because in that scenario the kernel will still start, just at the project root path. The problems with this are immediately apparent when you try and run a notebook nested in a folder. 

Now we run `URI.revive()` in `ExtHostLanguageRuntime.$createLanguageRuntimeSession()` if there's a `notebookUri` present to convert from the serialized version back to a real URI. 

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### QA Notes
The normal VSCode notebooks should be able to open resources relative to
themselves. Like a csv with `pd.read_csv()`

extensions/jupyter-adapter/src/JupyterKernel.ts

@@ -132,24 +132,16 @@ export class JupyterKernel extends EventEmitter implements vscode.Disposable {
 	private _receivedInitialStatus = false;
 	private _receivedInitialReply = false;
 
-	/** The URI of the notebook that owns this kernel, if any */
-	private readonly _notebookUri?: vscode.Uri;
-
 	constructor(
 		private readonly _context: vscode.ExtensionContext,
 		spec: JupyterKernelSpec,
 		private readonly _runtimeId: string,
 		private readonly _channel: vscode.OutputChannel,
-		readonly notebookUri?: vscode.Uri,
+		private readonly _notebookUri?: vscode.Uri,
 		readonly extra?: JupyterKernelExtra,
 	) {
 		super();
 
-		if (notebookUri) {
-			// Sometimes this uri comes in only partially formed, so we need to make sure its a full
-			// uri before we store it.
-			this._notebookUri = vscode.Uri.parse(notebookUri.toString());
-		}
 		this._spec = spec;
 		this._extra = extra;
 		this._control = null;

src/vs/workbench/api/common/positron/extHostLanguageRuntime.ts

@@ -84,6 +84,16 @@ export class ExtHostLanguageRuntime implements extHostProtocol.ExtHostLanguageRu
 		sessionMetadata: IRuntimeSessionMetadata): Promise<extHostProtocol.RuntimeInitialState> {
 		// Look up the session manager responsible for restoring this session
 		const sessionManager = await this.runtimeManagerForRuntime(runtimeMetadata, true);
+
+		if (sessionMetadata.notebookUri) {
+			// Sometimes the full URI doesn't make it across the serialization boundary.
+			// By reviving the URI here we make sure we're operating with a full URI
+			// rather than a serialized one that may be missing parameters.
+			sessionMetadata = {
+				...sessionMetadata,
+				notebookUri: URI.revive(sessionMetadata.notebookUri)
+			};
+		}
 		if (sessionManager) {
 			const session =
 				await sessionManager.manager.createSession(runtimeMetadata, sessionMetadata);