Fix BaseOrderedChannel: restore async Enqueue method.

[mikasoukhov]

This commit fixes critical issues in BaseOrderedChannel implementation:

1. Restored async signature of Enqueue method with CancellationToken support

   • Changed from synchronous blocking (AsyncHelper.Run) to proper async/await
   • This prevents thread blocking and deadlocks, especially in bounded channels
   • Allows cancellation of write operations

2. Fixed tests to use async Add method

   • Updated all test methods to properly await Add operations
   • Ensures tests correctly validate async behavior

3. Added clarifying documentation to Count property

   • Documented that Count only reflects items in sorted collection
   • Not including items still in the channel buffer

These changes fix potential deadlocks and restore proper async behavior.