Use KotlinTypeUtils.isKotlinUnit to match Kotlin void return types (#971)

Follow-up to #970, which disabled `doNotChangeAlreadyUnitTestMethods` with
comment "flaky on CI but I don't know why".

Root cause: as of rewrite 8.79.4 (openrewrite/rewrite#7364), the Kotlin
parser maps non-nullable `kotlin.Unit` to JVM `JavaType.Primitive.Void`.
`TypeUtils.isOfClassType(type, "kotlin.Unit")` returns false for a
primitive, so the recipe no longer skips already-Unit methods and rewrites
them unnecessarily — tripping the single-cycle stability check on CI.
Locally, stale `rewrite-kotlin` snapshots still carry the pre-change
behaviour, which is why the failure was CI-only.

Use `KotlinTypeUtils.isKotlinUnit` (introduced in the same PR #7364)
which accepts either `JavaType.Primitive.Void` or a `kotlin.Unit` class
reference, and re-enable the disabled test.

Author: timtebeek

src/main/java/org/openrewrite/java/testing/cleanup/KotlinTestMethodsShouldReturnUnit.java

@@ -26,6 +26,7 @@
 import org.openrewrite.kotlin.marker.KObject;
 import org.openrewrite.kotlin.marker.SingleExpressionBlock;
 import org.openrewrite.kotlin.tree.K;
+import org.openrewrite.kotlin.tree.KotlinTypeUtils;
 import org.openrewrite.marker.Markers;
 
 import static java.util.Collections.emptyList;
@@ -52,7 +53,7 @@ public J.MethodDeclaration visitMethodDeclaration(J.MethodDeclaration method, Ex
 
                 // Skip invalid signatures or already-correct return types
                 JavaType.Method methodType = m.getMethodType();
-                if (m.getBody() == null || methodType == null || TypeUtils.isOfClassType(methodType.getReturnType(), KOTLIN_UNIT.getFullyQualifiedName())) {
+                if (m.getBody() == null || methodType == null || KotlinTypeUtils.isKotlinUnit(methodType.getReturnType())) {
                     return m;
                 }
 

src/test/java/org/openrewrite/java/testing/cleanup/KotlinTestMethodsShouldReturnUnitTest.java

@@ -15,7 +15,6 @@
  */
 package org.openrewrite.java.testing.cleanup;
 
-import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 import org.openrewrite.DocumentExample;
 import org.openrewrite.InMemoryExecutionContext;
@@ -309,7 +308,6 @@ fun getValue() : String {
         );
     }
 
-    @Disabled("flaky on CI but I don't know why")
     @Test
     void doNotChangeAlreadyUnitTestMethods() {
         //language=kotlin