[spark] Close the commit after truncating a table or partitions

[cxzl25]

Purpose

Both Spark truncate paths create a BatchTableCommit and never close it:

• TruncatePaimonTableWithFilterExec.run() handles TRUNCATE TABLE with or
  without a partition spec on the driver.
• PaimonSparkTableBase.truncateTable implements Spark's TruncatableTable.

TableCommitImpl.close() shuts down the underlying commit and its
maintainExecutor thread pool, so every TRUNCATE TABLE leaked a thread pool
on the driver. Both call sites now wrap the commit in try/finally, matching
PaimonFormatTable.withCommit and the other commit call sites in this module.

Tests