Add explicit isDirectory flag to buildinfo.xml schema for robust artifact detection

Replaces path-based heuristics with an explicit boolean flag to determine
if a cached artifact is a directory (e.g., target/classes) or regular file.

Changes:
- build-cache-build.mdo: Added isDirectory field to Artifact class
- CacheControllerImpl.java:
  * artifactDto(): Set isDirectory=true for directory artifacts
  * restoreArtifactToDisk(): Use artifact.isIsDirectory() instead of path heuristics

Benefits:
- More robust: Works with any custom packaging type or path structure
- More maintainable: Single source of truth instead of maintaining heuristic rules
- Backward compatible: Missing flag defaults to false (regular file)
- Cleaner code: Replaced 10 lines of path-checking logic with 1-line boolean check

Example buildinfo.xml output:
  <artifact>
    ...
    <filePath>target/classes</filePath>
    <isDirectory>true</isDirectory>
  </artifact>

All tests passing: 74 unit tests + 26 integration tests

Author: cowwoc

src/main/java/org/apache/maven/buildcache/CacheControllerImpl.java

@@ -358,20 +358,9 @@ private UnaryOperator<File> createRestorationToDiskConsumer(final MavenProject p
      * Directory artifacts (cached as zips) are unzipped back to their original directory structure.
      */
     private void restoreArtifactToDisk(File cachedFile, Artifact artifact, Path restorationPath) throws IOException {
-        // Check if this is a directory artifact by examining the filePath from buildinfo.
-        // Directory artifacts have filePath like "target/classes" instead of "target/foo.jar"
-        // They are saved as zips but named with .jar extension (e.g., duplicate.jar)
-        String filePath = artifact.getFilePath();
-        boolean isDirectoryArtifact = filePath != null
-                && (filePath.equals("target/classes")
-                        || filePath.equals("target/test-classes")
-                        || (!filePath.endsWith(".jar")
-                                && !filePath.endsWith(".zip")
-                                && !filePath.endsWith(".war")
-                                && !filePath.endsWith(".ear")
-                                && !filePath.endsWith(".pom")));
-
-        if (isDirectoryArtifact) {
+        // Check the explicit isDirectory flag set during save.
+        // Directory artifacts (e.g., target/classes) are saved as zips and need to be unzipped on restore.
+        if (artifact.isIsDirectory()) {
             restoreDirectoryArtifact(cachedFile, artifact, restorationPath);
         } else {
             restoreRegularFileArtifact(cachedFile, artifact, restorationPath);
@@ -809,6 +798,9 @@ private Artifact artifactDto(
             if (Files.isRegularFile(file)) {
                 dto.setFileHash(algorithm.hash(file));
                 dto.setFileSize(Files.size(file));
+            } else if (Files.isDirectory(file)) {
+                // Mark directory artifacts explicitly so we can unzip them on restore
+                dto.setIsDirectory(true);
             }
 
             // Always set filePath (needed for artifact restoration)

src/main/mdo/build-cache-build.mdo

@@ -244,6 +244,12 @@ under the License.
           <name>filePath</name>
           <type>String</type>
         </field>
+        <!--xs:element name="isDirectory" type="xs:boolean" minOccurs="0"/-->
+        <field>
+          <name>isDirectory</name>
+          <type>boolean</type>
+          <description>Indicates if this artifact represents a directory (e.g., target/classes) that was zipped for caching</description>
+        </field>
       <!--/xs:sequence-->
       </fields>
     <!--/xs:complexType-->