Flaky specs in save_designs_service_spec.rb

What does this MR do and why?

This MR fixes a leaky example in save_designs_service_spec.rb. This was made more obvious after we merged additional tests impacted by the leak.

The leak: issue.reload doesn't have any effect on issue.design_collections.copy_state, because issue.design_collections is memoized. So the redis cache behavior is all fine, but copy_state leaks between examples through instance vars. That throws off the expected behavior for other examples.

MR acceptance checklist

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How to set up and validate locally

• Flaky specs in save_designs_service_spec.rb (#477821 - closed) includes repro instructions
• Alt repro: rspec spec/services/design_management/save_designs_service_spec.rb:148:168

Related to #477821 (closed)