feat: prefer native_datafusion scan in auto mode with fallback

In auto scan mode, try native_datafusion scan first and fall back to
native_iceberg_compat when native_datafusion is not supported (e.g.,
when COMET_EXEC_ENABLED is disabled, or DPP is used).

Author: andygrove

spark/src/main/scala/org/apache/comet/rules/CometScanRule.scala

@@ -167,7 +167,11 @@ case class CometScanRule(session: SparkSession)
         }
 
         COMET_NATIVE_SCAN_IMPL.get() match {
-          case SCAN_AUTO | SCAN_NATIVE_DATAFUSION =>
+          case SCAN_AUTO =>
+            nativeDataFusionScan(plan, session, scanExec, r, hadoopConf)
+              .orElse(nativeIcebergCompatScan(session, scanExec, r, hadoopConf))
+              .getOrElse(scanExec)
+          case SCAN_NATIVE_DATAFUSION =>
             nativeDataFusionScan(plan, session, scanExec, r, hadoopConf).getOrElse(scanExec)
           case SCAN_NATIVE_ICEBERG_COMPAT =>
             nativeIcebergCompatScan(session, scanExec, r, hadoopConf).getOrElse(scanExec)