[SPARK-38198][SQL][3.2] Fix QueryExecution.debug#toFile use the passed in maxFields when explainMode is CodegenMode

### What changes were proposed in this pull request?
`QueryExecution.debug#toFile` method supports passing in `maxFields` and this parameter will be passed down when `explainMode` is `SimpleMode`, `ExtendedMode`, or `CostMode`.

But the passed down `maxFields` was ignored when `explainMode` is `CostMode` because `QueryExecution#stringWithStats` overrides it with `SQLConf.get.maxToStringFields` at present,  so this pr removes the override behavior to let passed in `maxFields` take effect.

### Why are the changes needed?
Bug fix

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Pass GA and add a new test case

Closes #35515 from LuciferYang/SPARK-38198-32.

Authored-by: yangjie01 <[email protected]>
Signed-off-by: Max Gekk <[email protected]>

Author: LuciferYang

sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala

@@ -304,8 +304,6 @@ class QueryExecution(
   }
 
   private def stringWithStats(maxFields: Int, append: String => Unit): Unit = {
-    val maxFields = SQLConf.get.maxToStringFields
-
     // trigger to compute stats for logical plans
     try {
       // This will trigger to compute stats for all the nodes in the plan, including subqueries,

sql/core/src/test/scala/org/apache/spark/sql/execution/QueryExecutionSuite.scala

@@ -264,4 +264,22 @@ class QueryExecutionSuite extends SharedSparkSession {
     assert(cmdResultExec.commandPhysicalPlan.asInstanceOf[ExecutedCommandExec]
       .cmd.isInstanceOf[ShowTablesCommand])
   }
+
+  test("SPARK-38198: check specify maxFields when call toFile method") {
+    withTempDir { dir =>
+      val path = dir.getCanonicalPath + "/plans.txt"
+      // Define a dataset with 6 columns
+      val ds = spark.createDataset(Seq((0, 1, 2, 3, 4, 5), (6, 7, 8, 9, 10, 11)))
+      // `CodegenMode` and `FormattedMode` doesn't use the maxFields, so not tested in this case
+      Seq(SimpleMode.name, ExtendedMode.name, CostMode.name).foreach { modeName =>
+        val maxFields = 3
+        ds.queryExecution.debug.toFile(path, explainMode = Some(modeName), maxFields = maxFields)
+        Utils.tryWithResource(Source.fromFile(path)) { source =>
+          val tableScan = source.getLines().filter(_.contains("LocalTableScan"))
+          assert(tableScan.exists(_.contains("more fields")),
+            s"Specify maxFields = $maxFields doesn't take effect when explainMode is $modeName")
+        }
+      }
+    }
+  }
 }