Prevent marker-only changes from producing phantom diffs (#7358)

Dependency-modifying recipes now update JavaSourceSet markers (added in
#7202), which can produce Results with identical before/after text. This
caused downstream test failures in rewrite-dropwizard, rewrite-hibernate,
rewrite-migrate-java, and rewrite-spring.

Three fixes:

1. JavaSourceSet.addTypesForGav returns `this` when the gavKey already
   exists with the same types, preventing unnecessary allocations for
   LSTs with empty gavToTypes maps.

2. The cached overload of JavaSourceSet.updateOnSourceFile now checks
   whether the source file's current marker is already the cached
   instance before replacing it, preventing O(n) phantom diffs across
   files in the same source set.

3. RewriteTest accepts marker-only changes (identical printed text)
   instead of failing with "An empty diff was generated", since these
   represent legitimate internal state updates.

Closes #7349

Author: steve-aom-elliott

rewrite-java/src/main/java/org/openrewrite/java/marker/JavaSourceSet.java

@@ -66,6 +66,11 @@ public class JavaSourceSet implements SourceSet {
      * @return a new JavaSourceSet with the types added
      */
     public JavaSourceSet addTypesForGav(String gavKey, List<JavaType.FullyQualified> types) {
+        List<JavaType.FullyQualified> existing = gavToTypes.get(gavKey);
+        if (existing != null && existing.equals(types)) {
+            return this;
+        }
+
         List<JavaType.FullyQualified> newClasspath = new ArrayList<>(classpath);
         newClasspath.addAll(types);
 
@@ -178,6 +183,9 @@ public static SourceFile updateOnSourceFile(SourceFile sf, Map<String, JavaSourc
         String cacheKey = maybeJp.get().getId().toString() + ":" + maybeSourceSet.get().getName();
         JavaSourceSet cached = cache.get(cacheKey);
         if (cached != null) {
+            if (cached == maybeSourceSet.get()) {
+                return sf;
+            }
             return sf.withMarkers(sf.getMarkers().setByType(cached));
         }
         JavaSourceSet updated = transform.apply(maybeSourceSet.get());

rewrite-test/src/main/java/org/openrewrite/test/RewriteTest.java

@@ -562,7 +562,15 @@ default void rewriteRun(Consumer<RecipeSpec> spec, SourceSpec<?>... sourceSpecs)
                             boolean isRemote = result.getAfter() instanceof Remote;
                             if (!isRemote && Objects.equals(result.getBefore().getSourcePath(), result.getAfter().getSourcePath()) &&
                                 Objects.equals(before, actualAfter)) {
-                                fail("An empty diff was generated. The recipe incorrectly changed a reference without changing its contents.");
+                                // Marker-only change (e.g. updated JavaSourceSet classpath) — no visible diff.
+                                // Still call afterRecipe so marker assertions work, then skip.
+                                allResults.remove(result);
+                                try {
+                                    //noinspection unchecked
+                                    ((Consumer<SourceFile>) sourceSpec.afterRecipe).accept(result.getAfter());
+                                } catch (ClassCastException ignored) {
+                                }
+                                continue nextSourceFile;
                             }
 
                             assert result.getBefore() != null;