Assert integration test assets are configured

[KSemenenko]

Summary

• ensure the model integration test asserts native assets are present instead of skipping
• remove the SkipException dependency so missing assets surface as test failures

Testing

• dotnet build

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https://chatgpt.com/codex/tasks/task_e_68fc8959815c8326976b375ed6a4a2be

[Copilot]

Pull Request Overview

This PR converts the model integration test from a skip-on-missing-assets approach to a fail-on-missing-assets approach. The test now asserts that required native assets (model path and library) are properly configured rather than silently skipping when they're absent.

• Replaces Skip.If() calls with Assert.False() and Assert.True() statements
• Renames EnsureAssetsOrSkip() to EnsureAssets() to reflect the new behavior
• Adds detailed error messages explaining which environment variables need to be set

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