IEmbeddedLanguageClassificationService is optional in the workspaces …

…layer

The default implementation of IEmbeddedLanguageClassificationService is
provided in the features layer, so make sure it is consumed as optional
in the workspaces layer.

Fixes dotnet#63921

src/Workspaces/Core/Portable/Classification/Classifier.cs

@@ -77,7 +77,6 @@ internal static IEnumerable<ClassifiedSpan> GetClassifiedSpans(
  {
  var projectServices = services.GetLanguageServices(semanticModel.Language);
  var classificationService = projectServices.GetRequiredService<ISyntaxClassificationService>();
- var embeddedLanguageService = projectServices.GetRequiredService<IEmbeddedLanguageClassificationService>();
 
  var syntaxClassifiers = classificationService.GetDefaultSyntaxClassifiers();
 
@@ -94,8 +93,12 @@ internal static IEnumerable<ClassifiedSpan> GetClassifiedSpans(
  classificationService.AddSemanticClassifications(semanticModel, textSpan, getNodeClassifiers, getTokenClassifiers, semanticClassifications, options, cancellationToken);
 
  // intentionally adding to the semanticClassifications array here.
- if (includedEmbeddedClassifications && project != null)
+ if (includedEmbeddedClassifications
+ && project != null
+ && projectServices.GetService<IEmbeddedLanguageClassificationService>() is { } embeddedLanguageService)
+ {
  embeddedLanguageService.AddEmbeddedLanguageClassifications(services, project, semanticModel, textSpan, options, semanticClassifications, cancellationToken);
+ }
 
  var allClassifications = new List<ClassifiedSpan>(semanticClassifications.Where(s => s.TextSpan.OverlapsWith(textSpan)));
  var semanticSet = semanticClassifications.Select(s => s.TextSpan).ToSet();