Fix ArgumentDisplayFormatter ignored due to premature display name caching

[Copilot]

Custom ArgumentDisplayFormatter attributes were ignored because the display name was computed and cached before discovery event receivers ran, where formatters are registered.

public class Foo
{
    public override string ToString() => throw new Exception("Should not be called");
}

[Test]
[MethodDataSource(nameof(Data))]
[ArgumentDisplayFormatter<FooFormatter>]  // Was being ignored
public void Test(Foo foo) { }

Changes

• TUnit.Core/TestContext.cs: Added InvalidateDisplayNameCache() to clear cached display name
• TUnit.Engine/Building/TestBuilder.cs: Invalidate cache after discovery event receivers complete in BuildTestAsync()
• TUnit.TestProject/ArgumentDisplayFormatterTests.cs: Added test coverage for formatters with MethodDataSource, Arguments, and exception-throwing ToString()

Flow

Before: GetDisplayName() → cache result → discovery events → formatters registered (too late)

After: GetDisplayName() → cache result → discovery events → formatters registered → invalidate cache → next GetDisplayName() uses formatters

Original prompt

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This section details on the original issue you should resolve

<issue_title>ArgumentDisplayFormatter is being ignored in 0.77.3</issue_title>
<issue_description>A custom ArgumentDisplayFormatter is being ignored. Here's a minimal reproduction with a failing test because of it (here's a repo with the repro):

public class Tests
{
    [Test]
    [MethodDataSource(nameof(Data1))]
    [ArgumentDisplayFormatter<FooFormatter>]
    public void Test1(Foo foo)
    {
    }

    public IEnumerable<Foo> Data1() => [new()];
}

public class Foo
{
    public override string ToString() => throw new Exception("Foo.ToString");
}

public class FooFormatter : ArgumentDisplayFormatter
{
    public override bool CanHandle(object? value) => true;

    public override string FormatValue(object? value) => "FooFormatterValue";
}

The test fails because the FooFormatter isn't being used and the Foo.ToString() method gets called and throws:

PS D:\Projects\others\TUnitBugs> dotnet test
Restore complete (0.4s)
  TUnitBugs succeeded (0.2s) → TUnitBugs\bin\Debug\net9.0\TUnitBugs.dll
  TUnitBugs test failed with 2 error(s) (0.4s)
    D:\\Projects\\others\\TUnitBugs\\TUnitBugs\\Tests.cs(3): error test failed: 
      Test1 (0ms): InvalidOperationException: Failed to expand data source for test 'Test1': Foo.ToString
      
    D:\Projects\others\TUnitBugs\TUnitBugs\bin\Debug\net9.0\TUnitBugs.dll : error run failed: Tests failed: 'D:\Projects\others\TUnitBugs\TUnitBugs\bin\Debug\net9.0\TestResults\TUnitBugs_net9.0_x64.log' [net9.0|x64]

Test summary: total: 1, failed: 1, succeeded: 0, skipped: 0, duration: 0.3s
Build failed with 2 error(s) in 1.1s

I found that this happens because the argument display value is computed before the formatter is added.

The argument display value is being created here:

TUnit/TUnit.Engine/Building/TestBuilder.cs

Lines 771 to 783 in 6ed6416

	// Create the test object BEFORE invoking event receivers
	// This ensures context.InternalExecutableTest is set for error handling in registration
	var creationContext = new ExecutableTestCreationContext
	{
	TestId = testId,
	DisplayName = context.GetDisplayName(),
	Arguments = testData.MethodData,
	ClassArguments = testData.ClassData,
	Context = context,
	TestClassInstanceFactory = testData.TestClassInstanceFactory,
	ResolvedMethodGenericArguments = testData.ResolvedMethodGenericArguments,
	ResolvedClassGenericArguments = testData.ResolvedClassGenericArguments
	};

Which happens before the events are executed for the test later:

TUnit/TUnit.Engine/Building/TestBuilder.cs

Line 803 in 6ed6416

await InvokeDiscoveryEventReceiversAsync(context);

And ArgumentDisplayFormatterAttribute depends on having it's OnTestDiscovered executed so it could add the argument formatter.

I discovered this, because I have a class that throws for the default argument formatter, so I'm using a custom one to make sure my tests don't fail. Except right now the tests are failing anyways.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ArgumentDisplayFormatter is being ignored in 0.77.3 #3542

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