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/AbCipDriver.cs

@@ -134,7 +134,8 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
                             TagPath: $"{tag.TagPath}.{member.Name}",
                             DataType: member.DataType,
                             Writable: member.Writable,
-                            WriteIdempotent: member.WriteIdempotent);
+                            WriteIdempotent: member.WriteIdempotent,
+                            StringLength: member.StringLength);
                         _tagsByName[memberTag.Name] = memberTag;
                     }
                 }
@@ -633,7 +634,8 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
             CipPath: device.ParsedAddress.CipPath,
             LibplctagPlcAttribute: device.Profile.LibplctagPlcAttribute,
             TagName: parsed.ToLibplctagName(),
-            Timeout: _options.Timeout));
+            Timeout: _options.Timeout,
+            StringMaxCapacity: def.DataType == AbCipDataType.String ? def.StringLength : null));
         try
         {
             await runtime.InitializeAsync(ct).ConfigureAwait(false);