Replace custom error notification with standard LSP error messages

[wiyota]

Replace the custom ERROR_MESSAGE notification type with the standard LSP window.showErrorMessage API. This simplifies the error handling flow by:

• Removing the custom NotifyErrorParams interface and ERROR_MESSAGE notification type from connectionTypes.ts
• Removing the client-side notification handler in extension.ts
• Adding a showErrorMessage helper function in server handlers
• Updating error handling to use connection.window.showErrorMessage directly instead of sending custom notifications

This change will enable error messages to be displayed in editors other than VS Code.