Fix: Enable Polyfill package by default for netstandard2.0 in TUnit.Core

[Copilot]

The TUnit.Core package was not automatically pulling in the Polyfill package for consuming test projects targeting netstandard2.0, requiring users to manually add the package or explicitly set EnableTUnitPolyfills=true.

Problem

When developers created test projects targeting netstandard2.0 and referenced TUnit.Core, they would encounter missing method exceptions or compilation errors because modern .NET APIs used by TUnit were not available in netstandard2.0 without the Polyfill package.

Solution

Modified the condition in TUnit.Core.targets from:

Condition="'$(EnableTUnitPolyfills)' == 'true' and '$(TargetFramework)' == 'netstandard2.0'"

To:

Condition="'$(EnableTUnitPolyfills)' != 'false' and '$(TargetFramework)' == 'netstandard2.0'"

This change makes the Polyfill package included by default for netstandard2.0 targets while preserving the ability to opt out by setting <EnableTUnitPolyfills>false</EnableTUnitPolyfills> in the project file.

Testing

• Verified Polyfill package is automatically included for netstandard2.0 projects
• Confirmed opt-out behavior works correctly when EnableTUnitPolyfills is set to false
• Ensured net8.0+ projects don't get polyfills (existing correct behavior)
• Validated no breaking changes to existing functionality

Fixes #2744.

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