feat(s7): expose ReadMultipleVarsAsync through the IS7Plc seam

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7/IS7Plc.cs

@@ -1,4 +1,5 @@
 using S7.Net;
+using S7DataItem = global::S7.Net.Types.DataItem;
 using S7NetDataType = global::S7.Net.DataType;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.S7;
@@ -43,6 +44,22 @@ public interface IS7Plc : IDisposable
     /// <returns>The raw byte block.</returns>
     Task<byte[]> ReadBytesAsync(S7NetDataType area, int db, int startByteAdr, int count, CancellationToken cancellationToken);
 
+    /// <summary>
+    ///     Reads several individually-addressed items in ONE PDU round-trip — the multi-var request
+    ///     the span planner's leftovers batch onto instead of issuing an address read apiece.
+    /// </summary>
+    /// <remarks>
+    ///     ⚠️ <b>The CALLER owns chunking.</b> S7.Net does NOT split a multi-var request: both the item
+    ///     count and the total payload are bounded by the negotiated PDU size, and an oversized request
+    ///     fails on the wire rather than being paged. Split the item list to PDU capacity before calling.
+    ///     The library populates each <see cref="S7DataItem.Value"/> in place and hands the same list
+    ///     back; the returned list is the authoritative result and is ordered as supplied.
+    /// </remarks>
+    /// <param name="items">The items to read; each carries its area / DB / offset / var-type and receives its value.</param>
+    /// <param name="cancellationToken">Cancellation token for the read.</param>
+    /// <returns>The read items, in the supplied order, with their values populated.</returns>
+    Task<IReadOnlyList<S7DataItem>> ReadMultipleVarsAsync(List<S7DataItem> items, CancellationToken cancellationToken);
+
     /// <summary>Writes a scalar value by S7 address string; the value must be the unsigned wire type for the address size.</summary>
     /// <param name="address">The S7 address string.</param>
     /// <param name="value">The wire-typed value to write.</param>
@@ -132,6 +149,17 @@ internal sealed class S7PlcAdapter : IS7Plc
             t => _plc.ReadBytesAsync(area, db, startByteAdr, count, t),
             _operationTimeout, $"block read of {area} db{db} @{startByteAdr}+{count}", cancellationToken);
 
+    /// <inheritdoc />
+    public async Task<IReadOnlyList<S7DataItem>> ReadMultipleVarsAsync(List<S7DataItem> items, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(items);
+        // S7.Net returns the SAME list with each item's Value populated; await + widen because
+        // Task<List<T>> is not assignable to Task<IReadOnlyList<T>>.
+        return await S7OperationDeadline.RunAsync(
+            t => _plc.ReadMultipleVarsAsync(items, t),
+            _operationTimeout, $"multi-var read of {items.Count} item(s)", cancellationToken).ConfigureAwait(false);
+    }
+
     /// <inheritdoc />
     public Task WriteAsync(string address, object value, CancellationToken cancellationToken) =>
         S7OperationDeadline.RunAsync(

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7DriverReconnectTests.cs

@@ -569,6 +569,10 @@ public sealed class S7DriverReconnectTests
         public Task<byte[]> ReadBytesAsync(S7NetDataType area, int db, int startByteAdr, int count, CancellationToken cancellationToken) =>
             throw new NotSupportedException("FakeS7Plc: block reads not needed for the reconnect tests");
 
+        public Task<IReadOnlyList<global::S7.Net.Types.DataItem>> ReadMultipleVarsAsync(
+            List<global::S7.Net.Types.DataItem> items, CancellationToken cancellationToken) =>
+            throw new NotSupportedException("FakeS7Plc: multi-var reads not needed for the reconnect tests");
+
         public async Task WriteAsync(string address, object value, CancellationToken cancellationToken)
         {
             if (WriteHangsUntilCancelled)

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7PollEngineMigrationTests.cs

@@ -47,6 +47,10 @@ public sealed class S7PollEngineMigrationTests
             return Task.FromResult(block);
         }
 
+        public Task<IReadOnlyList<global::S7.Net.Types.DataItem>> ReadMultipleVarsAsync(
+            List<global::S7.Net.Types.DataItem> items, CancellationToken ct)
+            => throw new NotSupportedException("ConstantArrayPlc: multi-var reads not needed for the poll-engine tests");
+
         public Task WriteAsync(string address, object value, CancellationToken ct) => Task.CompletedTask;
         public Task WriteBytesAsync(S7NetDataType area, int db, int startByteAdr, byte[] value, CancellationToken ct) => Task.CompletedTask;
         public Task ReadStatusAsync(CancellationToken ct) => Task.CompletedTask;

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7SpanReadParityTests.cs

@@ -323,6 +323,11 @@ public sealed class S7SpanReadParityTests
                 return Task.FromResult(image[startByteAdr..(startByteAdr + count)]);
             }
 
+            /// <inheritdoc />
+            public Task<IReadOnlyList<global::S7.Net.Types.DataItem>> ReadMultipleVarsAsync(
+                List<global::S7.Net.Types.DataItem> items, CancellationToken ct)
+                => throw new NotSupportedException("ImageS7Plc: multi-var reads not served by the parity image fake");
+
             /// <inheritdoc />
             public Task<object?> ReadAsync(string address, CancellationToken ct)
             {

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7TestBuilders.cs

@@ -92,6 +92,10 @@ internal sealed class ConnectingFakeS7PlcFactory : IS7PlcFactory
         public Task<byte[]> ReadBytesAsync(S7NetDataType area, int db, int startByteAdr, int count, CancellationToken ct)
             => throw new NotSupportedException("ConnectingFakeS7Plc serves no data");
         /// <summary>Not exercised by these tests.</summary>
+        public Task<IReadOnlyList<global::S7.Net.Types.DataItem>> ReadMultipleVarsAsync(
+            List<global::S7.Net.Types.DataItem> items, CancellationToken ct)
+            => throw new NotSupportedException("ConnectingFakeS7Plc serves no data");
+        /// <summary>Not exercised by these tests.</summary>
         public Task WriteAsync(string address, object value, CancellationToken ct)
             => throw new NotSupportedException("ConnectingFakeS7Plc serves no data");
         /// <summary>Not exercised by these tests.</summary>