Add polyfills for ArgumentNullException.ThrowIfNull(...), ArgumentException.ThrowIfNullOrEmpty(...) and ArgumentException.ThrowIfNullOrWhiteSpace(...)

[Tyrrrz]

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[Copilot]

Pull Request Overview

This PR adds polyfills for three argument validation helper methods introduced in .NET 6.0 and .NET 7.0: ArgumentNullException.ThrowIfNull, ArgumentException.ThrowIfNullOrEmpty, and ArgumentException.ThrowIfNullOrWhiteSpace. These polyfills enable projects targeting older .NET versions to use these modern convenience methods for argument validation.

Key changes:

• Added ArgumentNullException.ThrowIfNull polyfill for .NET versions prior to 6.0
• Added ArgumentException.ThrowIfNullOrEmpty and ArgumentException.ThrowIfNullOrWhiteSpace polyfills for .NET versions prior to 7.0
• Comprehensive test coverage for all three methods verifying correct exception types and parameter names

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
PolyShim/Net60/ArgumentNullException.cs	Implements ThrowIfNull static method polyfill for .NET < 6.0, using CallerArgumentExpression to capture parameter names
PolyShim/Net70/ArgumentException.cs	Implements ThrowIfNullOrEmpty and ThrowIfNullOrWhiteSpace static method polyfills for .NET < 7.0, with proper null vs empty/whitespace exception handling
PolyShim.Tests/Net60/ArgumentNullExceptionTests.cs	Tests ThrowIfNull with null and non-null values, verifying correct exception and parameter name capture
PolyShim.Tests/Net70/ArgumentExceptionTests.cs	Tests both ThrowIfNullOrEmpty and ThrowIfNullOrWhiteSpace with appropriate test cases for null, empty/whitespace, and valid values

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