feat(tracing): add async context propagation helpers for JoinableTaskFactory

[CalvinAllen]

Summary

Activity.Current can be lost when switching threads in Visual Studio's async model, particularly when using JoinableTaskFactory. Add helpers to properly flow tracing context across async boundaries.

Problem

using (VsixTelemetry.StartActivity("ParentOperation"))
{
    await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync();
    // Activity.Current may be null here!
    
    using (VsixTelemetry.StartActivity("ChildOperation"))
    {
        // This span won't be linked to parent
    }
}

Proposed Solution

// Option 1: Extension method for JoinableTaskFactory
await ThreadHelper.JoinableTaskFactory
    .WithActivityContext()
    .SwitchToMainThreadAsync();

// Option 2: Wrapper that captures and restores context
using (VsixTelemetry.PreserveContext())
{
    await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync();
    // Activity.Current is preserved
}

// Option 3: Run with explicit context
await VsixTelemetry.RunWithContextAsync(async () =>
{
    await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync();
    // Context flows correctly
});

Implementation Notes

• Capture Activity.Current before thread switch
• Restore it after the switch using Activity.SetCurrent()
• Consider using AsyncLocal<T> for automatic propagation
• Test with various VS async patterns