feat(documents): implement custom document viewer editor

[CalvinAllen]

Summary

• Add custom VS editor for viewing Couchbase documents in tabbed editor pane
• Double-click or right-click "Open Document" on document nodes to open viewer
• Display document ID, collection path (bucket.scope.collection), and CAS value in header
• Provide Refresh button to reload document content from Couchbase
• Provide Copy JSON button to copy formatted content to clipboard
• Use Consolas font for JSON content display

New Files

• Editors/DocumentEditorControl.xaml - XAML UI for document viewer
• Editors/DocumentEditorControl.xaml.cs - Code-behind with Refresh/Copy handlers
• Editors/DocumentEditorPane.cs - WindowPane hosting the control
• Editors/DocumentEditorFactory.cs - IVsEditorFactory for .cbjson files
• Services/DocumentEditorService.cs - Service to open documents in custom editor

Test plan

• Connect to a Couchbase cluster
• Expand to a collection with documents
• Double-click a document - verify custom editor opens in tab
• Verify document ID, collection path, and CAS are displayed correctly
• Verify JSON content is formatted and displayed
• Click Refresh - verify content reloads
• Click Copy JSON - verify content is copied to clipboard
• Right-click document and select "Open Document" - verify same behavior