Auto: ablegacy-6 — ST string verification + length guard

Verifies libplctag's GetString/SetString round-trips ST file strings (1-word
length prefix + 82 ASCII bytes) end-to-end through the driver, and adds a
client-side length guard so over-82-char writes return BadOutOfRange instead
of being silently truncated by libplctag.

- LibplctagLegacyTagRuntime.EncodeValue: throws ArgumentOutOfRangeException
  for >82-char String writes (StFileMaxStringLength constant).
- AbLegacyDriver.WriteAsync: catches ArgumentOutOfRangeException and maps to
  BadOutOfRange.
- AbLegacyStringEncodingTests: 16 unit tests covering empty / 41-char /
  82-char / embedded-NUL / non-ASCII reads + writes; over-length writes
  return BadOutOfRange and never call WriteAsync; both Slc500 and Plc5
  family paths exercised.

Closes #249

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs

@@ -237,6 +237,13 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
             {
                 results[i] = new WriteResult(AbLegacyStatusMapper.BadOutOfRange);
             }
+            catch (ArgumentOutOfRangeException)
+            {
+                // ST-file string writes exceeding the 82-byte fixed element. Surfaces from
+                // LibplctagLegacyTagRuntime.EncodeValue's length guard; mapped to BadOutOfRange so
+                // the OPC UA client sees a clean rejection rather than a silent truncation.
+                results[i] = new WriteResult(AbLegacyStatusMapper.BadOutOfRange);
+            }
             catch (Exception ex)
             {
                 results[i] = new WriteResult(AbLegacyStatusMapper.BadCommunicationError);