IcebergWriteStrategy ignores spark.comet.enabled, so the split-operator write plan survives the kill switch

[andygrove]

Describe the bug

IcebergWriteStrategy checks only spark.comet.write.iceberg.splitOperator.enabled (spark/src/main/scala/org/apache/comet/iceberg/IcebergWriteStrategy.scala:38), and the strategy is registered unconditionally (CometSparkSessionExtensions.scala:102). It does not check spark.comet.enabled or whether Comet is loaded.

Today the split flag defaults to false, so this has no effect. Once it defaults to true (#5644), a session with spark.comet.enabled=false would still plan Iceberg writes as Comet's IcebergWriteExec + IcebergCommitExec. The main kill switch would no longer restore Spark's own write plan, which is the first thing users try when they suspect Comet.

Steps to reproduce

spark.conf.set("spark.comet.enabled", "false")
spark.conf.set("spark.comet.write.iceberg.splitOperator.enabled", "true")
spark.sql("INSERT INTO iceberg_table SELECT ...").explain()
// plan contains IcebergCommitExec / IcebergWriteExec

Expected behavior

With spark.comet.enabled=false (or Comet not loaded), the strategy returns Nil and Spark plans its own AppendDataExec / ReplaceDataExec and so on. Gate it the same way CometRule gates conversion.

Additional context

This blocks flipping the split-operator default, the first step of the rollout in #5644. Found in an audit of the native Iceberg write path. Part of #5649.