Fix .NET Framework compatibility: Replace CancelAsync() with Cancel()

[Copilot]

The pipeline was failing on Windows/.NET Framework due to the use of CancellationTokenSource.CancelAsync() in the assertion builder, which is only available in .NET 8.0 and later.

Problem

The error occurred in TUnit.Assertions/AssertionBuilders/AssertionBuilder.cs at line 193:

await cts.CancelAsync();  // ❌ Not available in .NET Framework 4.7.2

This caused unobserved task exceptions and pipeline failures when running tests targeting net472.

Solution

Replaced the async cancellation with the synchronous method that's available across all target frameworks:

cts.Cancel();  // ✅ Available in all .NET versions

Testing

• ✅ All target frameworks build successfully (net472, net8.0, net9.0)
• ✅ All 346 assertion tests pass on .NET 9.0
• ✅ Public API tests pass with no breaking changes
• ✅ .NET Framework 4.7.2 builds without errors

This is a minimal, surgical fix that maintains full compatibility across all supported .NET versions while resolving the pipeline failure.

Fixes #2729.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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