IFirstTestInAssemblyEventReceiver is invoked twice before any tests run in 0.50.x

[candhub]

Description

In 0.50.x, IFirstTestInAssemblyEventReceiver.OnFirstTestInAssembly is invoked twice before any tests start, while [Before(Assembly)] runs exactly once. This reproduces with the minimal sample below. On 0.25.21 both were called exactly once.

Environment

• TUnit: 0.50.x (after Fix first event receiver thread safety using GetOnlyDictionary + Task pattern #2917 fix)
• macOS 15.5 (arm64)
• .NET: 9.0
• IDE: Rider

Test Code

using TUnit.Core;
using TUnit.CoreHooks;
using TUnit.Core.Tests;

namespace GNS.TUNIT_LAB;

public class CallEventReceiverTests : IFirstTestInAssemblyEventReceiver
{
    private static int tmp = 1;

    [Test]
    public async Task Test1()
    {
        var result = true;
        await Assert.That(result).IsTrue();
    }

    [Test]
    public async Task Test2()
    {
        var result = true;
        await Assert.That(result).IsTrue();
    }

    [Before(Assembly)]
    public static void Before_assembly()
    {
        tmp++;
        Console.WriteLine($"Before_assembly tmp={tmp}");
    }

    public ValueTask OnFirstTestInAssembly(AssemblyHookContext context, TestContext testContext)
    {
        tmp++;
        Console.WriteLine($"OnFirstTestInAssembly tmp={tmp}");
        return ValueTask.CompletedTask;
    }
}

Steps to Reproduce

1. Run the test class above with TUnit 0.50.x.
2. Observe console output or set breakpoints in both Before_assembly and OnFirstTestInAssembly.

Expected Behavior

• OnFirstTestInAssembly is called exactly once per assembly, and before any test runs.
• [Before(Assembly)] is also called exactly once, before tests.

Actual Behavior (0.50.x)

• OnFirstTestInAssembly is invoked twice before any tests start.
• [Before(Assembly)] is invoked once.
• This differs from 0.25.21, where each was invoked once.

Notes

• I will attach a GIF showing the two invocations of OnFirstTestInAssembly before test execution.
• This seems related to the thread-safety change in Fix first event receiver thread safety using GetOnlyDictionary + Task pattern #2917 but still results in duplicate assembly-first calls.

Requests

• Restore the exactly-once-per-assembly guarantee for IFirstTestInAssemblyEventReceiver before test execution.
• Clarify the documented ordering guarantees between OnFirstTestInAssembly and [Before(Assembly)].

[candhub]

This issue persists in version 0.57.24.
In the example code, the variable tmp should be incremented once by the Before_assembly() method, then incremented again by the OnFirstTestInAssembly() method, resulting in a value of 3.
However, the OnFirstTestInAssembly() method is still being called twice, causing the tmp variable to actually become 4.