[SPARK-54573][SQL] Add test for CONSTRAINTS in case of source with less nested struct field than target

sql/core/src/test/scala/org/apache/spark/sql/connector/MergeIntoTableSuiteBase.scala

@@ -5235,6 +5235,54 @@ abstract class MergeIntoTableSuiteBase extends RowLevelOperationSuiteBase
     sql(s"DROP TABLE IF EXISTS $tableNameAsString")
   }
 
+  test("merge with struct missing nested field with check constraint") {
+    withTempView("source") {
+      // Target table has struct with nested field c2
+      createAndInitTable(
+        s"""pk INT NOT NULL,
+           |s STRUCT<c1: INT, c2: INT>,
+           |dep STRING""".stripMargin,
+        """{ "pk": 0, "s": { "c1": 1, "c2": 10 }, "dep": "sales" }
+          |{ "pk": 1, "s": { "c1": 2, "c2": 20 }, "dep": "hr" }"""
+          .stripMargin)
+
+      // Add CHECK constraint on nested field c2 using ALTER TABLE
+      sql(s"ALTER TABLE $tableNameAsString ADD CONSTRAINT check_c2 CHECK " +
+        s"(s.c2 IS NOT NULL AND s.c2 > 1)")
+
+      // Source table schema with struct missing the c2 field
+      val sourceTableSchema = StructType(Seq(
+        StructField("pk", IntegerType),
+        StructField("s", StructType(Seq(
+          StructField("c1", IntegerType)
+          // missing field 'c2' which has CHECK constraint IS NOT NULL AND > 1
+        ))),
+        StructField("dep", StringType)
+      ))
+
+      val data = Seq(
+        Row(1, Row(100), "engineering"),
+        Row(2, Row(200), "finance")
+      )
+      spark.createDataFrame(spark.sparkContext.parallelize(data), sourceTableSchema)
+        .createOrReplaceTempView("source")
+
+      withSQLConf(SQLConf.MERGE_INTO_NESTED_TYPE_COERCION_ENABLED.key -> "true") {
+        val error = intercept[SparkRuntimeException] {
+          sql(
+            s"""MERGE INTO $tableNameAsString t USING source
+                |ON t.pk = source.pk
+                |WHEN MATCHED THEN
+                | UPDATE SET s = source.s, dep = source.dep
+                |""".stripMargin)}
+        assert(error.getCondition == "CHECK_CONSTRAINT_VIOLATION")
+        assert(error.getMessage.contains("CHECK constraint check_c2 s.c2 IS NOT NULL AND " +
+          "s.c2 > 1 violated by row with values:\n - s.c2 : null"))
+      }
+    }
+    sql(s"DROP TABLE IF EXISTS $tableNameAsString")
+  }
+
   test("merge with schema evolution using dataframe API: add new column and set all") {
     Seq(true, false).foreach { withSchemaEvolution =>
       val sourceTable = "cat.ns1.source_table"