Fix crash when classifying embedded test classifications (#76576)

Author: CyrusNajmabadi

src/EditorFeatures/CSharpTest/Classification/SemanticClassifierTests_TestMarkup.cs

@@ -20,7 +20,7 @@
 
 namespace Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.Classification;
 
-public partial class SemanticClassifierTests : AbstractCSharpClassifierTests
+public sealed partial class SemanticClassifierTests : AbstractCSharpClassifierTests
 {
     private const string s_testMarkup = """
 
@@ -53,6 +53,27 @@ void M()
         await TestEmbeddedCSharpWithMultipleSpansAsync(allCode, testHost, spans, expected);
     }
 
+    private async Task TestSingleLineEmbeddedCSharpAsync(
+       string code,
+       TestHost testHost,
+       params FormattedClassification[] expected)
+    {
+        var allCode = $$"""""
+            class C
+            {
+                void M()
+                {
+                    Test.M(""""{{code}}"""");
+                }
+            }
+            """"" + s_testMarkup;
+
+        var start = allCode.IndexOf(code, StringComparison.Ordinal);
+        var length = code.Length;
+        var spans = ImmutableArray.Create(new TextSpan(start, length));
+        await TestEmbeddedCSharpWithMultipleSpansAsync(allCode, testHost, spans, expected);
+    }
+
     private async Task TestEmbeddedCSharpWithMultipleSpansAsync(
        string allCode,
        TestHost testHost,
@@ -457,4 +478,26 @@ class D
             Punctuation.Semicolon,
             Punctuation.CloseCurly);
     }
+
+    [Theory, CombinatorialData]
+    [WorkItem("https://github.com/dotnet/roslyn/issues/76575")]
+    public async Task TestOnlyMarkup1(TestHost testHost)
+    {
+        await TestEmbeddedCSharpAsync(
+            "[||]",
+            testHost,
+            TestCodeMarkdown("[|"),
+            TestCodeMarkdown("|]"));
+    }
+
+    [Theory, CombinatorialData]
+    [WorkItem("https://github.com/dotnet/roslyn/issues/76575")]
+    public async Task TestOnlyMarkup2(TestHost testHost)
+    {
+        await TestSingleLineEmbeddedCSharpAsync(
+            "[||]",
+            testHost,
+            TestCodeMarkdown("[|"),
+            TestCodeMarkdown("|]"));
+    }
 }

src/Features/CSharp/Portable/EmbeddedLanguages/CSharpTestEmbeddedLanguageClassifier.cs

@@ -20,14 +20,10 @@ namespace Microsoft.CodeAnalysis.CSharp.Features.EmbeddedLanguages;
 [ExportEmbeddedLanguageClassifier(
     PredefinedEmbeddedLanguageNames.CSharpTest, [LanguageNames.CSharp], supportsUnannotatedAPIs: false,
     PredefinedEmbeddedLanguageNames.CSharpTest), Shared]
-internal sealed class CSharpTestEmbeddedLanguageClassifier : IEmbeddedLanguageClassifier
+[method: ImportingConstructor]
+[method: Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
+internal sealed class CSharpTestEmbeddedLanguageClassifier() : IEmbeddedLanguageClassifier
 {
-    [ImportingConstructor]
-    [Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
-    public CSharpTestEmbeddedLanguageClassifier()
-    {
-    }
-
     private static TextSpan FromBounds(VirtualChar vc1, VirtualChar vc2)
         => TextSpan.FromBounds(vc1.Span.Start, vc2.Span.End);
 
@@ -90,7 +86,7 @@ public void RegisterClassifications(EmbeddedLanguageClassificationContext contex
                 }
             }
         }
-        else
+        else if (!virtualCharsWithoutMarkup.IsEmpty)
         {
             context.AddClassification(
                 ClassificationTypeNames.TestCode,