[SPARK-51004][ML][PYTHON][CONNECT] Add supports for IndexString

### What changes were proposed in this pull request?
This PR add supports for IndexString and add labels/labelsArray to ALLOWED_LIST.

### Why are the changes needed?

new feature parity and bug fix

### Does this PR introduce _any_ user-facing change?
Yes

### How was this patch tested?
CI passes

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #49690 from wbo4958/index-str.

Authored-by: Bobby Wang <wbo4958@gmail.com>
Signed-off-by: Ruifeng Zheng <ruifengz@apache.org>
(cherry picked from commit b5deb8d)
Signed-off-by: Ruifeng Zheng <ruifengz@apache.org>

Author: wbo4958

mllib/src/main/resources/META-INF/services/org.apache.spark.ml.Transformer

@@ -32,6 +32,7 @@ org.apache.spark.ml.feature.SQLTransformer
 org.apache.spark.ml.feature.StopWordsRemover
 org.apache.spark.ml.feature.FeatureHasher
 org.apache.spark.ml.feature.HashingTF
+org.apache.spark.ml.feature.IndexToString
 
 ########### Model for loading
 # classification

python/pyspark/ml/tests/test_feature.py

@@ -72,6 +72,7 @@
     BucketedRandomProjectionLSHModel,
     MinHashLSH,
     MinHashLSHModel,
+    IndexToString,
 )
 from pyspark.ml.linalg import DenseVector, SparseVector, Vectors
 from pyspark.sql import Row
@@ -80,6 +81,51 @@
 
 
 class FeatureTestsMixin:
+    def test_index_string(self):
+        dataset = self.spark.createDataFrame(
+            [
+                (0, "a"),
+                (1, "b"),
+                (2, "c"),
+                (3, "a"),
+                (4, "a"),
+                (5, "c"),
+            ],
+            ["id", "label"],
+        )
+
+        indexer = StringIndexer(inputCol="label", outputCol="labelIndex").fit(dataset)
+        transformed = indexer.transform(dataset)
+        idx2str = (
+            IndexToString()
+            .setInputCol("labelIndex")
+            .setOutputCol("sameLabel")
+            .setLabels(indexer.labels)
+        )
+
+        def check(t: IndexToString) -> None:
+            self.assertEqual(t.getInputCol(), "labelIndex")
+            self.assertEqual(t.getOutputCol(), "sameLabel")
+            self.assertEqual(t.getLabels(), indexer.labels)
+
+        check(idx2str)
+
+        ret = idx2str.transform(transformed)
+        self.assertEqual(
+            sorted(ret.schema.names), sorted(["id", "label", "labelIndex", "sameLabel"])
+        )
+
+        rows = ret.select("label", "sameLabel").collect()
+        for r in rows:
+            self.assertEqual(r.label, r.sameLabel)
+
+        # save & load
+        with tempfile.TemporaryDirectory(prefix="index_string") as d:
+            idx2str.write().overwrite().save(d)
+            idx2str2 = IndexToString.load(d)
+            self.assertEqual(str(idx2str), str(idx2str2))
+            check(idx2str2)
+
     def test_dct(self):
         df = self.spark.createDataFrame([(Vectors.dense([5.0, 8.0, 6.0]),)], ["vec"])
         dct = DCT()
@@ -128,6 +174,7 @@ def test_string_indexer(self):
         si = StringIndexer(inputCol="label1", outputCol="index1")
         model = si.fit(df.select("label1"))
         self.assertEqual(si.uid, model.uid)
+        self.assertEqual(model.labels, list(model.labelsArray[0]))
 
         # read/write
         with tempfile.TemporaryDirectory(prefix="string_indexer") as tmp_dir:
@@ -188,6 +235,7 @@ def test_pca(self):
         pca = PCA(k=2, inputCol="features", outputCol="pca_features")
 
         model = pca.fit(df)
+        self.assertTrue(np.allclose(model.pc.toArray()[0], [-0.44859172, -0.28423808], atol=1e-4))
         self.assertEqual(pca.uid, model.uid)
         self.assertEqual(model.getK(), 2)
         self.assertTrue(

sql/connect/server/src/main/scala/org/apache/spark/sql/connect/ml/MLUtils.scala

@@ -646,6 +646,7 @@ private[ml] object MLUtils {
     (classOf[Word2VecModel], Set("getVectors", "findSynonyms", "findSynonymsArray")),
     (classOf[CountVectorizerModel], Set("vocabulary")),
     (classOf[OneHotEncoderModel], Set("categorySizes")),
+    (classOf[StringIndexerModel], Set("labels", "labelsArray")),
     (classOf[IDFModel], Set("idf", "docFreq", "numDocs")))
 
   private def validate(obj: Any, method: String): Unit = {