Closes #7274

Describe your changes:

The remove container button was showing up in the toolbar even when there was only one container left in the flexible layout. clicking it would either fail or delete the content inside wich is pretty confusing for users.

Added a check in toolbarProvider.js to only show the button when containers.length > 1. Also added an e2e test that verifies the button hides and shows correctly based on container count.

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Author Checklist

• Changes address original issue?
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