fix(site-runtime): TraceCapturingLogger rejects double-wrapping; ScriptLogger clip keeps surrogate pairs (Task 4 review)

src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Scripts/ScriptLogger.cs

@@ -165,7 +165,28 @@ public sealed class ScriptLogger
         }
     }
 
-    /// <summary>Applies the per-message cap, marking a truncated message with a trailing ellipsis.</summary>
-    private static string Clip(string? m) =>
-        m is null ? string.Empty : m.Length <= MaxMessageChars ? m : m[..MaxMessageChars] + "…";
+    /// <summary>
+    /// Applies the per-message cap, marking a truncated message with a trailing ellipsis.
+    ///
+    /// <para>Cuts one char short when the cap would land between the two halves of a surrogate
+    /// pair, so an emoji straddling the boundary is dropped whole rather than leaving a lone
+    /// surrogate that breaks JSON serialization downstream. Re-clipping an already-clipped message
+    /// is a no-op, which matters because the script path clips once before logging and again
+    /// before emitting.</para>
+    /// </summary>
+    private static string Clip(string? m)
+    {
+        if (m is null)
+        {
+            return string.Empty;
+        }
+
+        if (m.Length <= MaxMessageChars)
+        {
+            return m;
+        }
+
+        var keep = char.IsHighSurrogate(m[MaxMessageChars - 1]) ? MaxMessageChars - 1 : MaxMessageChars;
+        return m[..keep] + "…";
+    }
 }