fix(xml): reject malformed else-if instead of silently dropping

Author: RoboNET

src/FlexRender.Xml/XmlToYamlNodeConverter.cs

@@ -202,11 +202,14 @@ private static void AddWrapper(YamlMappingNode node, string parentType, XElement
                 node.Add(name, ConvertElementSequence(wrapper));
                 break;
             case "else-if":
-                var inner = wrapper.Elements().FirstOrDefault();
-                if (inner is not null)
+                var children = wrapper.Elements().ToList();
+                if (children.Count != 1
+                    || !string.Equals(children[0].Name.LocalName, "if", StringComparison.Ordinal))
                 {
-                    node.Add("elseIf", ConvertElement(inner));
+                    throw new TemplateParseException(
+                        "An <else-if> must contain exactly one <if> child element.");
                 }
+                node.Add("elseIf", ConvertElement(children[0]));
                 break;
             case "columns":
                 node.Add("columns", ConvertAttributeItemSequence(wrapper));

tests/FlexRender.Tests/Parsing/Xml/XmlControlFlowTests.cs

@@ -1,3 +1,4 @@
+using FlexRender.Parsing;
 using FlexRender.Parsing.Ast;
 using FlexRender.Xml;
 using Xunit;
@@ -78,4 +79,64 @@ public void Parse_If_ElseIf()
         Assert.NotNull(ifEl.ElseIf);
         Assert.Equal("warm", ifEl.ElseIf!.CompareValue);
     }
+
+    [Fact]
+    public void Parse_ElseIf_WithNoChild_Throws()
+    {
+        const string xml = """
+            <flexrender>
+              <canvas width="300"/>
+              <if condition="status" equals="hot">
+                <then><text content="HOT"/></then>
+                <else-if></else-if>
+              </if>
+            </flexrender>
+            """;
+
+        var ex = Assert.Throws<TemplateParseException>(() => _parser.Parse(xml));
+        Assert.Contains("else-if", ex.Message);
+    }
+
+    [Fact]
+    public void Parse_ElseIf_WithNonIfChild_Throws()
+    {
+        const string xml = """
+            <flexrender>
+              <canvas width="300"/>
+              <if condition="status" equals="hot">
+                <then><text content="HOT"/></then>
+                <else-if>
+                  <text content="oops"/>
+                </else-if>
+              </if>
+            </flexrender>
+            """;
+
+        var ex = Assert.Throws<TemplateParseException>(() => _parser.Parse(xml));
+        Assert.Contains("else-if", ex.Message);
+    }
+
+    [Fact]
+    public void Parse_ElseIf_WithMultipleChildren_Throws()
+    {
+        const string xml = """
+            <flexrender>
+              <canvas width="300"/>
+              <if condition="status" equals="hot">
+                <then><text content="HOT"/></then>
+                <else-if>
+                  <if condition="status" equals="warm">
+                    <then><text content="WARM"/></then>
+                  </if>
+                  <if condition="status" equals="cool">
+                    <then><text content="COOL"/></then>
+                  </if>
+                </else-if>
+              </if>
+            </flexrender>
+            """;
+
+        var ex = Assert.Throws<TemplateParseException>(() => _parser.Parse(xml));
+        Assert.Contains("else-if", ex.Message);
+    }
 }