feat(s7): String (S7 STRING) scalar read+write via S7.Net.Types.S7String

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7/S7Driver.cs

@@ -618,8 +618,11 @@ public sealed class S7Driver
             S7DataType.UInt64 => System.Buffers.Binary.BinaryPrimitives.ReadUInt64BigEndian(block),
             S7DataType.Float64 => System.Buffers.Binary.BinaryPrimitives.ReadDoubleBigEndian(block),
 
-            S7DataType.String => throw new NotSupportedException(
-                "S7 String scalar reads land in a follow-up PR"),
+            // S7 classic STRING: [maxLen byte][curLen byte][chars…]. S7.Net's S7String.FromByteArray
+            // reads the two-byte header and returns exactly curLen ASCII chars, ignoring the reserved
+            // padding past curLen — it validates curLen against the block, so no extra guard is needed.
+            S7DataType.String => global::S7.Net.Types.S7String.FromByteArray(block),
+
             S7DataType.DateTime => throw new NotSupportedException(
                 "S7 DateTime scalar reads land in a follow-up PR"),
 
@@ -662,7 +665,13 @@ public sealed class S7Driver
             }
 
             case S7DataType.String:
-                throw new NotSupportedException("S7 String scalar writes land in a follow-up PR");
+                // S7.Net's S7String.ToByteArray builds [maxLen=StringLength][curLen][chars…] and pads
+                // the result to the full reserved field (StringLength + 2 bytes) — exactly the width
+                // ReadScalarBlockAsync read, so WriteBytesAsync writes the whole reserved STRING. A value
+                // longer than StringLength throws ArgumentException (S7.Net rejects overflow; we do NOT
+                // silently truncate). A null value encodes as the empty string.
+                return global::S7.Net.Types.S7String.ToByteArray(Convert.ToString(value) ?? "", tag.StringLength);
+
             case S7DataType.DateTime:
                 throw new NotSupportedException("S7 DateTime scalar writes land in a follow-up PR");