🧪 Add missing test for AsyncButtonSpinner strokeWidth

[google-labs-jules]

🎯 What: Adds a missing test to ensure AsyncButtonSpinner correctly propagates strokeWidth to CircularProgressIndicator.
📊 Coverage: A new test case checks that an explicit strokeWidth value correctly appears on the rendered CircularProgressIndicator.
✨ Result: Improved test coverage for AsyncButtonSpinner properties, making future refactoring safer and explicitly testing its visual properties.

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PR created automatically by Jules for task 2895533043407522700 started by @esenmx

[google-labs-jules]

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