Add test coverage for Task/ValueTask-returning methods with __arglist

[Copilot]

Adds test cases validating that Task and ValueTask<int> returning methods work correctly with __arglist when invoked both directly and via await.

Test coverage:

• Direct calls to methods with __arglist that return Task/ValueTask
• Await operator with methods using __arglist that return Task/ValueTask
• Validation that fixed and variable arguments are correctly passed through when calling Task/ValueTask-returning methods

[MethodImpl(MethodImplOptions.NoInlining)]
private static Task TaskWithArglist(int fixedArg, __arglist)
{
    ArgIterator args = new ArgIterator(__arglist);
    int arg1 = __refvalue(args.GetNextArg(), int);
    int arg2 = __refvalue(args.GetNextArg(), int);
    // Validate args...
    
    return Task.CompletedTask;
}

// Test both direct call and await
TaskWithArglist(42, __arglist(100, 200)).Wait();
await TaskWithArglist(42, __arglist(100, 200));

Files added:

• src/tests/async/varargs/varargs.cs - Test implementation
• src/tests/async/varargs/varargs.csproj - Project file

Note: The methods with __arglist are not async themselves - they are synchronous methods that return completed Task or ValueTask<int> values.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[RuntimeAsync] Add a testcase for Task-returning methods that use __arglst</issue_title>
<issue_description>We need to add a testcase under src\tests\async\varargs .
The testcase should contain scenarios validating that both direct calls and await operator work correctly with methods that use __arglist feature in cases when the methods return:

• Task
• ValueTask<int>

</issue_description>

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

• Fixes [RuntimeAsync] Add a testcase for Task-returning methods that use __arglst #122499

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[VSadov]

@copilot the methods with __arglist do not need to be async themselves. Just returning a completed Task or ValueTask<int> is sufficient.

[Copilot]

@copilot the methods with __arglist do not need to be async themselves. Just returning a completed Task or ValueTask<int> is sufficient.

Updated in e52ac733. The methods with __arglist now return completed Task.CompletedTask and ValueTask without being async.