Refactor transports to help enable graceful shutdown (#142)

- This also paves the way for better multi-session support
- We should definitely rethink names for the transport API
- For now, I kept the names similar as possible, so we can
  focus on the API shape

Co-authored-by: Stephen Toub <stoub@microsoft.com>

tests/ModelContextProtocol.Tests/EverythingSseServerFixture.cs

@@ -56,8 +56,6 @@ public class EverythingSseServerFixture : IAsyncDisposable
             // Log the exception but don't throw
             await Console.Error.WriteLineAsync($"Error stopping Docker container: {ex.Message}");
         }
-
-        GC.SuppressFinalize(this);
     }
 
     private static bool CheckIsDockerAvailable()