[SPARK-53660][SQL][TESTS] Add unit test for Metadata equality check

### What changes were proposed in this commit?
This commit introduces a new unit test to verify the equality check of `StructField` instances with different metadata configurations.

### Why are the changes needed?
To ensure that the equality check correctly identifies `StructField` instances with empty and non-empty metadata as unequal.

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

No

### How was this patch tested?

Added a new unit test.

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

No

Closes #52397 from Yicong-Huang/ES-1587692/fix-add-test.

Authored-by: Yicong-Huang <17627829+Yicong-Huang@users.noreply.github.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

Author: Yicong-Huang

sql/catalyst/src/test/scala/org/apache/spark/sql/types/MetadataSuite.scala

@@ -79,6 +79,18 @@ class MetadataSuite extends SparkFunSuite {
     intercept[NoSuchElementException](meta.getLong("no_such_key"))
   }
 
+  test("Metadata equality check") {
+
+    // Create a StructField with empty metadata
+    val field1 = StructField("myField", IntegerType, nullable = true, Metadata.empty)
+
+    // Create a StructField with non-empty metadata
+    val metadata = new MetadataBuilder().putString("description", "An integer field").build()
+    val field2 = StructField("myField", IntegerType, nullable = true, metadata)
+
+    assert(!(field1 == field2), s"field1 = $field1, field2 = $field2")
+  }
+
   test("Kryo serialization for expressions") {
     val conf = new SparkConf()
     val serializer = new KryoSerializer(conf).newInstance()