test: enable Comet shuffle manager in CometBenchmarkBase

Author: andygrove

spark/src/test/scala/org/apache/spark/sql/benchmark/CometBenchmarkBase.scala

@@ -50,6 +50,12 @@ trait CometBenchmarkBase
       .setAppName("CometReadBenchmark")
       // Since `spark.master` always exists, overrides this value
       .set("spark.master", "local[1]")
+      // Required for native Comet shuffle (CometShuffleExchangeExec); must be set
+      // at session start. Without this, shuffles fall back to JVM Spark and break
+      // the native chain across exchanges.
+      .set(
+        "spark.shuffle.manager",
+        "org.apache.spark.sql.comet.execution.shuffle.CometShuffleManager")
       .setIfMissing("spark.driver.memory", "3g")
       .setIfMissing("spark.executor.memory", "3g")