[SPARK-52870][SQL] Properly quote variable names in FOR statement

### What changes were proposed in this pull request?

This PR fixes the bugs presented in https://issues.apache.org/jira/browse/SPARK-52870 . Before this change, we passed FOR statement referenced column names as multipart identifiers to `SetVariable`. This fails when the columns are not formatted as proper multipart identifiers. This PR changes the logic to always quote the column names, similar to how backticks are used from the SQL side.

### Why are the changes needed?

Bugfix.

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

No.

### How was this patch tested?

Unit tests added.

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

No.

Closes apache#51560 from dusantism-db/for-quote-variable-names.

Authored-by: Dušan Tišma <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: dusantism-db

sql/core/src/main/scala/org/apache/spark/sql/scripting/SqlScriptingExecutionNode.scala

@@ -1187,7 +1187,7 @@ class ForStatementExec(
       OneRowRelation()
     )
     val setIdentifierToCurrentRow =
-      SetVariable(Seq(UnresolvedAttribute(varName)), projectNamedStruct)
+      SetVariable(Seq(UnresolvedAttribute.quoted(varName)), projectNamedStruct)
     new SingleStatementExec(
       setIdentifierToCurrentRow,
       Origin(),

sql/core/src/test/scala/org/apache/spark/sql/scripting/SqlScriptingExecutionSuite.scala

@@ -2938,4 +2938,76 @@ class SqlScriptingExecutionSuite extends QueryTest with SharedSparkSession {
     )
     verifySqlScriptResult(sqlScript, expected = expected)
   }
+
+  test("Integer literal column in FOR query") {
+    val sqlScript1 =
+      """
+        |BEGIN
+        |  FOR SELECT 1 DO
+        |    SELECT 2;
+        |  END FOR;
+        |END
+        |""".stripMargin
+    verifySqlScriptResult(sqlScript1, Seq(Seq(Row(2))))
+
+    val sqlScript2 =
+      """
+        |BEGIN
+        |  FOR x AS SELECT 1 DO
+        |    SELECT x.`1`;
+        |  END FOR;
+        |END
+        |""".stripMargin
+    verifySqlScriptResult(sqlScript2, Seq(Seq(Row(1))))
+  }
+
+  test("Column with space in FOR query") {
+    val sqlScript1 =
+      """
+        |BEGIN
+        |  FOR SELECT 1 AS `Space Column` DO
+        |    SELECT `Space Column`;
+        |  END FOR;
+        |END
+        |""".stripMargin
+    val expected = Seq(Seq(Row(1)))
+    verifySqlScriptResult(sqlScript1, expected)
+
+    val sqlScript2 =
+      """
+        |BEGIN
+        |  FOR x AS SELECT 1 AS `Space Column` DO
+        |    SELECT x.`Space Column`;
+        |  END FOR;
+        |END
+        |""".stripMargin
+    verifySqlScriptResult(sqlScript2, expected)
+  }
+
+  test("FOR query referencing table with space") {
+    withTable("test_tbl") {
+      sql("CREATE TABLE test_tbl (`Space Column` INT) USING parquet")
+      sql("INSERT INTO test_tbl VALUES (1)")
+      val sqlScript1 =
+      """
+        |BEGIN
+        |  FOR SELECT * FROM test_tbl DO
+        |    SELECT `Space Column`;
+        |  END FOR;
+        |END
+        |""".stripMargin
+      val expected = Seq(Seq(Row(1)))
+      verifySqlScriptResult(sqlScript1, expected)
+
+      val sqlScript2 =
+      """
+        |BEGIN
+        |  FOR x AS SELECT * FROM test_tbl DO
+        |    SELECT x.`Space Column`;
+        |  END FOR;
+        |END
+        |""".stripMargin
+      verifySqlScriptResult(sqlScript2, expected)
+    }
+  }
 }