Fix NCalc type error when evaluating AI-generated formulas

NCalc2 operates on double internally; passing decimal parameters caused
'Operator * cannot be applied to double and decimal' at runtime.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Infrastructure/Services/CustomFormulaAiService.cs

@@ -181,7 +181,7 @@ Rules:
             foreach (var kv in variables)
             {
                 expr.Parameters[kv.Key] = kv.Value.ValueKind == JsonValueKind.Number
-                    ? (object)kv.Value.GetDecimal()
+                    ? (object)kv.Value.GetDouble()
                     : (object)(kv.Value.GetString() ?? "");
             }