[SPARK-59202][ML][TESTS] Enable save/load round-trip tests in Java ML tree suites

[uros-b]

What changes were proposed in this pull request?

Six Java ML tree test suites -- JavaDecisionTreeClassifierSuite, JavaGBTClassifierSuite, JavaRandomForestClassifierSuite, JavaDecisionTreeRegressorSuite, JavaGBTRegressorSuite, JavaRandomForestRegressorSuite -- carried commented-out save/load round-trip tests with a // TODO: Add test once save/load are implemented. SPARK-6725 marker.

Save/load has long shipped (MLWritable / MLReadable). This enables those tests. The commented code was obsolete -- it used the old save(SparkContext, path) / load(SparkContext, path) mllib API and referenced stale variable names (model2 / model3) -- so each block is rewritten to the current API:

File tempDir = Utils.createTempDir(System.getProperty("java.io.tmpdir"), "spark");
String path = new File(tempDir, "model").getPath();
try {
  model.save(path);
  <Model> sameModel = <Model>.load(path);
  TreeTests.checkEqual(model, sameModel);
} finally {
  Utils.deleteRecursively(tempDir);
}

Why are the changes needed?

The blocks were dead (obsolete API and wrong variable names) rather than merely disabled. Enabling them adds Java-facing save/load round-trip coverage for the tree models, matching what the Scala suites already provide via DefaultReadWriteTest.

Does this PR introduce any user-facing change?

No. Test-only.

How was this patch tested?

Ran the six enabled Java suites locally; all pass.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Opus 4.8)

[uros-b]

Thank you @tdcmeehan! Adding @HyukjinKwon & @zhengruifeng for ML