🧪 [testing improvement] Add test for Mutate<T> struct extension

[johnstrand]

🎯 What: Added tests for the untesed public method Mutate for structs in ObjectExtensions.cs.
📊 Coverage: Added tests for the happy path and the edge case where the action is null.
✨ Result: Increased test coverage and confidence in the Mutate struct extension.

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PR created automatically by Jules for task 467257177823890378 started by @johnstrand

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