Fix build error: Update test to use ToJson instead of ToJsonAsync

The serialization API changed after the merge with develop branch.
Updated the test to use the new synchronous ToJson() method instead
of the obsolete ToJsonAsync() method.

Co-authored-by: alexzautke <548617+alexzautke@users.noreply.github.com>

Author: Copilot

src/Hl7.Fhir.Specification.Shared.Tests/Source/TerminologyTests.cs

@@ -787,7 +787,7 @@ public async Tasks.Task ExpandedValueSetShouldSerializeSuccessfully()
             var resource = await server.Expand(parameters);
 
             // This should not throw a NullReferenceException
-            var json = await resource.ToJsonAsync();
+            var json = resource.ToJson();
 
             // Verify the expansion was successful
             Assert.NotNull(json);