Handle Post() failure in TwoWayAgent.Tell() and remove unused using

- Fault or cancel the TaskCompletionSource when ActionBlock.Post() returns
  false, preventing callers from hanging indefinitely on a completed/canceled agent.
- Remove unused System.Threading using in test file.

Author: fw2568

src/Dbosoft.Functional/Agent.cs

@@ -138,9 +138,16 @@ await process(state, t.Item1)
 
         public Task<TReply> Tell(TMsg message)
         {
-            var tcs = new TaskCompletionSource<TReply>(TaskCreationOptions.RunContinuationsAsynchronously);  
-             _actionBlock.Post((message, tcs));
-             
+            var tcs = new TaskCompletionSource<TReply>(TaskCreationOptions.RunContinuationsAsynchronously);
+
+            if (!_actionBlock.Post((message, tcs)))
+            {
+                if (_cancellationToken.IsCancellationRequested)
+                    tcs.SetCanceled();
+                else
+                    tcs.SetException(new InvalidOperationException("The agent is no longer accepting messages."));
+            }
+
             return tcs.Task;
         }
     }

test/Dbosoft.Functional.Tests/TwoWayAgentTests.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Threading;
 using System.Threading.Tasks;
 using Dbosoft.Functional;
 using Xunit;