Auto: abcip-1.2 — STRINGnn variant decoding

Closes #226

Adds nullable StringLength to AbCipTagDefinition + AbCipStructureMember
so STRING_20 / STRING_40 / STRING_80 UDT variants decode against the
right DATA-array capacity. The configured length threads through a new
StringMaxCapacity field on AbCipTagCreateParams and lands on the
libplctag Tag.StringMaxCapacity attribute (verified property on
libplctag 1.5.2). Null leaves libplctag's default 82-byte STRING in
place for back-compat. Driver gates on DataType == String so a stray
StringLength on a DINT tag doesn't reshape that buffer. UDT member
fan-out copies StringLength from the AbCipStructureMember onto the
synthesised member tag definition.

Tests: 4 new in AbCipDriverReadTests covering threaded StringMaxCapacity,
the null back-compat path, the non-String gate, and the UDT-member fan-out.

src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/IAbCipTagRuntime.cs

@@ -65,10 +65,15 @@ public interface IAbCipTagFactory
 /// <param name="LibplctagPlcAttribute">libplctag <c>plc=...</c> attribute, per family profile.</param>
 /// <param name="TagName">Logix symbolic tag name as emitted by <see cref="AbCipTagPath.ToLibplctagName"/>.</param>
 /// <param name="Timeout">libplctag operation timeout (applies to Initialize / Read / Write).</param>
+/// <param name="StringMaxCapacity">Optional Logix STRINGnn DATA-array capacity (e.g. 20 / 40 / 80
+///     for <c>STRING_20</c> / <c>STRING_40</c> / <c>STRING_80</c> UDTs). Threads through libplctag's
+///     <c>str_max_capacity</c> attribute. <c>null</c> keeps libplctag's default 82-byte STRING
+///     behaviour for back-compat.</param>
 public sealed record AbCipTagCreateParams(
     string Gateway,
     int Port,
     string CipPath,
     string LibplctagPlcAttribute,
     string TagName,
-    TimeSpan Timeout);
+    TimeSpan Timeout,
+    int? StringMaxCapacity = null);