🛡️ Sentinel: [MEDIUM] Add input validation to Duck<T> extension

[timonkrebs]

🚨 Severity: MEDIUM
💡 Vulnerability: The Duck<T> extension method lacked input validation for the instance parameter. If a null instance was passed, the code fell through to a confusing InvalidOperationException claiming no proxy was generated, instead of failing fast with a proper exception.
🎯 Impact: This could lead to confusing stack traces and masked null reference issues downstream, violating the principle of failing securely and predictably.
🔧 Fix: Added ArgumentNullException.ThrowIfNull(instance); at the beginning of the Duck<T> method to explicitly validate the input.
✅ Verification: Ran dotnet test successfully, ensuring the method's behavior is correct without breaking existing tests.

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PR created automatically by Jules for task 216805755355021586 started by @timonkrebs

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Pull request overview

This PR hardens the Duck<T> runtime fallback in NTypeForge by adding an explicit null check for the extension-method receiver, ensuring null inputs fail fast with a clear exception instead of falling through to a misleading InvalidOperationException.

Changes:

• Added ArgumentNullException.ThrowIfNull(instance); at the start of Duck<T> to validate the instance parameter.

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