src/vs/workbench/api/common/extHostNotebook.ts/ExtHostNotebookController/openNotebookDocument
class ExtHostNotebookController:	async openNotebookDocument(uri: URI): Promise<vscode.NotebookDocument> {
		const cached = this._documents.get(uri);
		if (cached) {
			return cached.apiNotebook;
		}
		const canonicalUri = await this._notebookDocumentsProxy.$tryOpenNotebook(uri);
		const document = this._documents.get(URI.revive(canonicalUri));
		return assertIsDefined(document?.apiNotebook);
	}