feat(mtconnect): agent-client observation write (HTTP POST, AllowPut surface)

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.Tests/CannedAgentClient.cs

@@ -355,6 +355,32 @@ internal sealed class CannedAgentClient : IMTConnectAgentClient, IDisposable
         }
     }
 
+    /// <summary>
+    ///     Every write this Agent was asked to take, in order — the record a driver-level write test
+    ///     asserts against (which DataItem, on which device, carrying what).
+    /// </summary>
+    public ConcurrentQueue<(string DeviceName, string DataItemId, string Value)> Writes { get; } = new();
+
+    /// <summary>
+    ///     What <see cref="WriteObservationAsync"/> answers with. Defaults to acceptance; set it to a
+    ///     refusal to script the Agent's <c>MTConnectError</c> leg.
+    /// </summary>
+    public MTConnectWriteOutcome WriteOutcome { get; set; } = new(Accepted: true, AgentError: null);
+
+    /// <summary>When set, a write throws this instead of answering — the transport-failure leg.</summary>
+    public Exception? WriteFailure { get; set; }
+
+    /// <inheritdoc/>
+    public Task<MTConnectWriteOutcome> WriteObservationAsync(
+        string deviceName, string dataItemId, string value, CancellationToken cancellationToken)
+    {
+        ObjectDisposedException.ThrowIf(IsDisposed, this);
+        cancellationToken.ThrowIfCancellationRequested();
+        Writes.Enqueue((deviceName, dataItemId, value));
+
+        return WriteFailure is null ? Task.FromResult(WriteOutcome) : throw WriteFailure;
+    }
+
     /// <summary>
     ///     Queues a chunk onto the scripted <c>/sample</c> stream and waits until the consumer has
     ///     processed it. This is the deterministic replacement for "wait a bit and hope the pump