fix: hide remove container btn when only one container exists

[dcpagotto]

Summary

• the remove container btn was showing up in the toolbar even
  when there was only one container left in flexible layout
• clicking it would fail or delete the content inside wich is
  confusing for users
• added a check in toolbarProvider.js to only show the btn when
  containers.length > 1
• also added e2e test that verifies the btn hides/shows correctly

Test plan

• e2e test added covering the exact scenario
• Start openmct, create a flexible layout
• In edit mode, remove containers until one remains
• Verify "Remove Container" button is not in the toolbar
• Add a container back, verify the button reappears

Closes #7274

[dcpagotto]

Hey, just checking in on this one. Is there anything else I should change or add to get this reviewed? I tried to keep the changes minimal and added the e2e test to cover the scenario. Let me know if theres something missing or if you need me to rebase on main.