🧪 [Testing Improvement] Add NUnit test project and tests for Identification.GetModel

[ManupaKDU]

🎯 What: The testing gap addressed
Added a comprehensive NUnit test suite for the GetModel function in Identification.cs, covering a range of inputs mapped to the Model enum.

📊 Coverage: What scenarios are now tested
Tests cover standard models (e.g., '880GMH/USB3', 'Z390 AORUS ULTRA'), unexpected string values (returns Model.Unknown), OEM default strings ('To be filled by O.E.M.'), and null input handling.

✨ Result: The improvement in test coverage
Introduced an entirely new test project (OpenHardwareMonitor.Tests), allowing robust testing for GetModel and creating a reliable framework base for any future .NET 8 test additions in the legacy .NET 4.5 project.

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PR created automatically by Jules for task 4775522215395930268 started by @manupawickramasinghe

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