Add tests for default attachedOutputs behavior

Tests verify that:
- When attachedOutputs is not configured, it defaults to 'classes' and 'test-classes'
- When attachedOutputs is explicitly configured, it overrides the defaults

Addresses reviewer feedback from PR apache#388 requesting tests for restoration behavior.

Author: claude

src/test/java/org/apache/maven/buildcache/xml/CacheConfigImplTest.java

@@ -31,6 +31,7 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Optional;
 import java.util.Properties;
@@ -39,7 +40,9 @@
 import org.apache.commons.lang3.tuple.Pair;
 import org.apache.maven.buildcache.DefaultPluginScanConfig;
 import org.apache.maven.buildcache.hash.HashFactory;
+import org.apache.maven.buildcache.xml.config.AttachedOutputs;
 import org.apache.maven.buildcache.xml.config.Configuration;
+import org.apache.maven.buildcache.xml.config.DirName;
 import org.apache.maven.buildcache.xml.config.Remote;
 import org.apache.maven.execution.MavenExecutionRequest;
 import org.apache.maven.execution.MavenSession;
@@ -482,4 +485,45 @@ void testRemoveSaveFinalIgnoredWhenRemoteSaveDisabled() {
                 Pair.of("getUrl", () -> assertEquals("dummy.url.xyz", testObject.getUrl())),
                 Pair.of("isRemoteCacheEnabled", () -> assertTrue(testObject.isRemoteCacheEnabled())));
     }
+
+    @Test
+    void testDefaultAttachedOutputsWhenNotConfigured() {
+        // When attachedOutputs is not configured in XML, should return default list
+        Configuration configuration = new Configuration();
+        // Deliberately not setting attachedOutputs
+        testCacheConfig.setConfiguration(configuration);
+
+        assertEquals(CacheState.INITIALIZED, testObject.initialize());
+
+        List<DirName> attachedOutputs = testObject.getAttachedOutputs();
+        assertEquals(2, attachedOutputs.size(), "Should have 2 default attached outputs");
+
+        List<String> dirNames = attachedOutputs.stream()
+                .map(DirName::getValue)
+                .collect(Collectors.toList());
+
+        assertTrue(dirNames.contains("classes"), "Should include 'classes' directory by default");
+        assertTrue(dirNames.contains("test-classes"), "Should include 'test-classes' directory by default");
+    }
+
+    @Test
+    void testExplicitAttachedOutputsOverridesDefaults() {
+        // When attachedOutputs is explicitly configured, should use those values instead of defaults
+        Configuration configuration = new Configuration();
+        AttachedOutputs attachedOutputs = new AttachedOutputs();
+
+        DirName customDir = new DirName();
+        customDir.setValue("custom-output");
+        attachedOutputs.addDirName(customDir);
+
+        configuration.setAttachedOutputs(attachedOutputs);
+        testCacheConfig.setConfiguration(configuration);
+
+        assertEquals(CacheState.INITIALIZED, testObject.initialize());
+
+        List<DirName> result = testObject.getAttachedOutputs();
+        assertEquals(1, result.size(), "Should have 1 explicitly configured output");
+        assertEquals("custom-output", result.get(0).getValue(),
+                "Should use explicitly configured directory, not defaults");
+    }
 }