🧪 Add reference equality edge case tests for StringExtensions.Repeat

[johnstrand]

🎯 What: Added tests to verify fast paths (count=0, empty string, count=1) return expected string instances.
📊 Coverage: Covered edge case fast-paths for string.Empty and original string references.
✨ Result: Improved test reliability and ensured fast paths do not regress.

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PR created automatically by Jules for task 4487181848845106269 started by @johnstrand

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