🧪 Add missing error path test for MaxPlainTextSize in CryptoService

[Avicennasis]

🎯 What: The testing gap addressed: Missing Error Path Test for MaxPlainTextSize in CryptoService
📊 Coverage: What scenarios are now tested: Encrypting a string exceeding 10MB now correctly asserts ArgumentException is thrown.
✨ Result: The improvement in test coverage: Increased confidence and code coverage by covering the DoS protection branch in CryptoService.Encrypt.

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PR created automatically by Jules for task 8324001220755888630 started by @Avicennasis

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