Add failing test for ArgumentOutOfRangeException in tag helper resolution

When an orphan end tag (e.g. </input> without a matching start tag)
immediately follows HTML text content and the tag name matches a legacy
tag helper, DefaultTagHelperResolutionPhase throws
ArgumentOutOfRangeException.

Root cause: the legacy resolver's ConvertToPlainElement removes the
wrapper at parent.Children[index], inserts the lowered HTML for the
orphan end tag, then calls MergeAdjacentHtmlContent, which merges those
tokens into the preceding text sibling and removes one item from
parent.Children. The caller then indexes parent.Children[index] using
the now-stale index.

This is the same class of bug fixed by PR #83516, just in a different
code path.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people

src/Compiler/Microsoft.AspNetCore.Razor.Language/test/DefaultRazorTagHelperBinderPhaseTest.cs

@@ -495,6 +495,45 @@ public void Execute_CombinesErrorsOnRewritingErrors()
         Assert.Equal<RazorDiagnostic>([initialError, expectedRewritingError], outputTree.Diagnostics);
     }
 
+    [Theory]
+    [InlineData("Hello</input>")]
+    [InlineData("Hello</input")]
+    [InlineData("text content</form>")]
+    [InlineData("foo</p>")]
+    public void Execute_OrphanEndTagAfterText_DoesNotThrow(string content)
+    {
+        var formTagHelper = CreateTagHelperDescriptor(
+            tagName: "form",
+            typeName: "TestFormTagHelper",
+            assemblyName: "TestAssembly");
+        var inputTagHelper = CreateTagHelperDescriptor(
+            tagName: "input",
+            typeName: "TestInputTagHelper",
+            assemblyName: "TestAssembly");
+        var pTagHelper = TagHelperDescriptorBuilder.CreateTagHelper("pTagHelper", "TestAssembly")
+            .TagMatchingRuleDescriptor(rule =>
+                rule
+                .RequireTagName("p")
+                .RequireAttributeDescriptor(attribute => attribute.Name("class")))
+            .Build();
+
+        var projectEngine = RazorProjectEngine.Create(builder =>
+        {
+            builder.SetTagHelpers(formTagHelper, inputTagHelper, pTagHelper);
+        });
+
+        var source = TestRazorSourceDocument.Create("@addTagHelper *, TestAssembly\r\n" + content, filePath: null);
+        var codeDocument = projectEngine.CreateCodeDocument(source);
+        var originalTree = RazorSyntaxTree.Parse(source);
+        codeDocument = codeDocument.WithSyntaxTree(originalTree);
+
+        codeDocument = projectEngine.ExecutePhase<DefaultRazorTagHelperContextDiscoveryPhase>(codeDocument);
+        codeDocument = projectEngine.ExecutePhase<DefaultRazorIntermediateNodeLoweringPhase>(codeDocument);
+        codeDocument = projectEngine.ExecutePhase<DefaultTagHelperResolutionPhase>(codeDocument);
+
+        Assert.NotNull(codeDocument.GetDocumentNode());
+    }
+
     private static string AssemblyA => "TestAssembly";
 
     private static string AssemblyB => "AnotherAssembly";