test: cover native Iceberg write handoff cleanup

[sam-1112]

Which issue does this PR close?

Closes part of #5646 (native-to-JVM handoff cleanup).

Rationale for this change

A native Iceberg writer can successfully return its task payload, then fail on the JVM while decoding the manifest and building the task commit message. At that point, no commit message reaches the driver, so the task's data files must be deleted by its failure listener.

What changes are included in this PR?

• Add a scoped test-only failpoint after WrittenFileCleanup owns native written_file_locations and before manifest decoding.
• Add an end-to-end native Iceberg write regression test that injects a post-native handoff failure.
• Verify all failed task attempts report non-empty native payload locations, no snapshot is committed, and no failed-task Parquet file remains on disk or in Iceberg metadata.

How are these changes tested?

make core

./mvnw test -Pspark-3.5 -Dtest=none \
  '-Dsuites=org.apache.comet.CometIcebergWriteActionSuite post-native handoff failure'

./mvnw test -Dtest=none \
  '-Dsuites=org.apache.comet.CometIcebergWriteActionSuite post-native handoff failure'

./mvnw spotless:check -Pspark-3.5

The targeted regression passes on Spark 3.5 and the default Spark 4.1 profile.