feat(language-service): provide diagnostics for invalid styleUrls (#32674)

Similar to diagnostics for invalid templateUrls, check that styleUrls
actually point to a valid Url.

Closes #32564.

PR Close #32674

packages/language-service/test/diagnostics_spec.ts

@@ -507,6 +507,37 @@ describe('diagnostics', () => {
           diagnostics.find(d => d.messageText === 'URL does not point to a valid file');
       expect(urlDiagnostic).toBeUndefined();
     });
+
+    it('should report errors for invalid styleUrls', () => {
+      const fileName = mockHost.addCode(`
+        @Component({
+          styleUrls: ['«notAFile»'],
+        })
+        export class MyComponent {}`);
+
+      const marker = mockHost.getReferenceMarkerFor(fileName, 'notAFile');
+
+      const diagnostics = ngLS.getDiagnostics(fileName) !;
+      const urlDiagnostic =
+          diagnostics.find(d => d.messageText === 'URL does not point to a valid file');
+      expect(urlDiagnostic).toBeDefined();
+
+      const {start, length} = urlDiagnostic !;
+      expect(start).toBe(marker.start);
+      expect(length).toBe(marker.length);
+    });
+
+    it('should not report errors for valid styleUrls', () => {
+      const fileName = '/app/app.component.ts';
+      mockHost.override(fileName, `
+        @Component({
+          styleUrls: ['./test.css', './test.css'],
+        })
+        export class MyComponent {}`);
+
+      const diagnostics = ngLS.getDiagnostics(fileName) !;
+      expect(diagnostics.length).toBe(0);
+    });
   });
 
   // https://github.com/angular/vscode-ng-language-service/issues/235