Resolve #956

This change fixes an intermittent IOException in Crypt32Tests.ShouldTimestampData where the test attempted to read the test timestamp server's log file before it was fully closed. This eliminates the race condition where tests could access files still held open by the timestamp service.

• Made RespondAsync(...) truly async with async Task and await in finally block
• Created async WriteAsync(...) method using await using and async write operations