fix(triggers): bound expression evaluation, align AlarmActor error handling, dedupe config parsing

src/ScadaLink.SiteRuntime/Scripts/TriggerExpressionGlobals.cs

@@ -1,3 +1,5 @@
+using System.Text.Json;
+
 namespace ScadaLink.SiteRuntime.Scripts;
 
 /// <summary>
@@ -11,6 +13,29 @@ namespace ScadaLink.SiteRuntime.Scripts;
 /// </summary>
 public sealed class TriggerExpressionGlobals
 {
+    /// <summary>
+    /// Extracts the <c>"expression"</c> field from an Expression-trigger config
+    /// JSON document. Returns <c>null</c> for a missing, blank, or malformed
+    /// config — the single parsing idiom shared by InstanceActor, ScriptActor,
+    /// and AlarmActor.
+    /// </summary>
+    public static string? ExtractExpression(string? triggerConfigJson)
+    {
+        if (string.IsNullOrEmpty(triggerConfigJson)) return null;
+        try
+        {
+            using var doc = JsonDocument.Parse(triggerConfigJson);
+            var expr = doc.RootElement.TryGetProperty("expression", out var e)
+                ? e.GetString()
+                : null;
+            return string.IsNullOrWhiteSpace(expr) ? null : expr;
+        }
+        catch (JsonException)
+        {
+            return null;
+        }
+    }
+
     private readonly IReadOnlyDictionary<string, object?> _snapshot;
 
     public TriggerExpressionGlobals(IReadOnlyDictionary<string, object?> snapshot)