Add AVRepro console app for reproducing arm64 AccessViolationException

[Copilot]

Adds a test harness to reproduce AccessViolationException bugs (dotnet/runtime#123324, dotnet/runtime#112167) on arm64 macOS by spawning parallel processes using both standard and experimental APIs.

Changes

• AVRepro console app: Targets net10.0 with single-file publishing and compression enabled
  • No args: Spawns 8 parallel /bin/echo processes via Process.Start
  • With args: Same workload using ChildProcess.ExecuteAsync API
• CI extension: macOS leg now publishes for osx-arm64 and executes both modes 10 times in a loop to increase reproduction rate

// Example: stress test with new API
var tasks = Enumerable.Range(0, 8)
    .Select(i => Task.Run(async () =>
    {
        ProcessStartOptions options = new("/bin/echo")
        {
            Arguments = { "hello!" }
        };
        await ChildProcess.ExecuteAsync(options);
    }));
await Task.WhenAll(tasks);

The CI workflow runs both execution modes (with and without new API) 10 times sequentially to maximize the chances of reproducing the AccessViolationException issue.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Try to reproduce AccessViolationException on arm64</issue_title>
<issue_description>.NET runtime users are hunted by these two bugs: dotnet/runtime#123324 and dotnet/runtime#112167.

Let's do the following:

• Add new C# console app to the solution. Let's name it "AVRepro"
• Use following MSBuild settings:

    <PropertyGroup>
      <OutputType>Exe</OutputType>
      <TargetFramework>net10.0</TargetFramework>
      <SelfContained>true</SelfContained>
      <PublishSingleFile>true</PublishSingleFile>
      <EnableCompressionInSingleFile>true</EnableCompressionInSingleFile>
    </PropertyGroup>

• Add reference to Library.csproj project file
• Add a logic that runs the following code if no args were specified:

  using System.Diagnostics;

  var tasks = Enumerable.Range(0, 2)
      .Select(i => Task.Run(async () =>
      {
          var psi = new ProcessStartInfo
          {
              FileName = "/bin/echo",
              Arguments = "hello!",
          };
          using var process = Process.Start(psi)!;
          await process.WaitForExitAsync();
      }));

  await Task.WhenAll(tasks);

• Add a logic that uses the new API (ChildProcess.ExecuteAsync) to do the same when args were specified.

• Extend the macOS CI leg with logic that achieves the following:

  dotnet publish -c Release -r osx-arm64
  ./bin/Release/net10.0/osx-arm64/publish/AVRepro --useNewAPI
  ./bin/Release/net10.0/osx-arm64/publish/AVRepro

</issue_description>

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

• Fixes Try to reproduce AccessViolationException on arm64 #103

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[adamsitnik]

@copilot You did exactly what I asked for, great! Since the problem does not reproduce, we need to do it more than once. Please address my comment.

[adamsitnik]

let's introduce a loop and run the last two steps ten times

[adamsitnik]

@copilot please try one more time

[Copilot]

Added a loop to run both AVRepro executions 10 times in commit d52d32c.