Bug Description:

The vscode.window.showTextDocument method supports only Theia URIs, and is incompatible with the VSCode URI passed as an argument when an editor/context menu command is executed.

Steps to Reproduce:

1. Create a VS Code extension that contributes an editor/context menu command named theia.sampleCommand, and registers a handler like the following:

   vscode.commands.registerCommand("theia.sampleCommand", async (uri) => {
       // Ensure document is open and not in preview mode
       vscode.window.showTextDocument(uri, { preview: false });
       // Do other stuff
       vscode.window.showInformationMessage(uri.toString());
   });

2. Install the extension in Theia and trigger the command by clicking it in editor context menu.
3. Note that the info message never appears, and there is an error in the console:

   Uncaught (in promise) 
   TypeError: Cannot read properties of undefined (reading 'toString')
       at DocumentsExtImpl.showDocument (:3000/home/theia/nod…documents.js:192:63)
       at Object.showTextDocument (:3000/home/theia/nod…n-context.js:232:33)
       at Object.<anonymous> (:3000/home/theia/.th…ension.js:33:299442)
       at Generator.next (<anonymous>)
       at :3000/home/theia/.th…ension.js:33:276506
       at new Promise (<anonymous>)
       at r (:3000/home/theia/.th…ension.js:33:276251)
       at t.submitJcl (:3000/home/theia/.th…ension.js:33:299223)
       at Object.<anonymous> (:3000/home/theia/.th…ension.js:90:501340)
       at Generator.next (<anonymous>)
   

Additional Information

• Operating System: Alpine Linux (Docker)
• Theia Version: 1.49.1

I believe I've tracked this down to the following line in showTextDocument, which has an instanceof check for Theia URIs:

When editor/context menu command is executed, the argument passed is an instance of CodeUri which is populated by the getSelectedResources method:

Since the URI passed to the command handler is a VSCode URI (from @theia/core/shared/vscode-uri), it fails the instanceof check that expects a Theia URI, resulting in document.uri evaluating to undefined.