[spark] Close the commit and release the cached RDD in remove_unexisting_files

[cxzl25]

Purpose

SparkRemoveUnexistingFiles creates a TableCommit on the executor inside foreachPartition but never closes it. TableCommitImpl.close() shuts down the underlying commit and the maintainExecutor thread pool, so every invocation of the procedure leaks a thread pool on a long-lived executor. Other call sites such as PaimonPartitionManagement already wrap the commit in try/finally.

The intermediate RDD is also cached and never unpersisted. buildRDD() returned a child of the cached RDD to the caller, so the cache could only be released after the caller finished, which never happened. This change turns it into execute(), which collects the result inside the method and unpersists the cache in a finally block.

Behaviour of the procedure is unchanged; only resource lifecycle is affected.

Tests