[spark] Support deleted record metrics for V2 delete

[kerwin-zk]

Purpose

This PR adds a number of deleted records commit metric for V2 copy-on-write DELETE. The V2 row-level operation path only applies to append tables without deletion vectors or data evolution, so the file-level row counts are exact and the number of deleted records is exactly:

deletedRecords = rowCount(removed files) - rowCount(rewritten files)

The value is computed at commit time from the commit messages (no extra IO) and posted through the existing driver metrics mechanism.

For UPDATE and MERGE, the rewritten files mix copied rows with modified rows, so record-level counts cannot be derived from file statistics and remain unreported.

Tests

Extended PaimonMetricTest ("Paimon Metric: delete") to assert deletedRecords for both a partial-file delete and a whole-file delete.

[JingsongLi]

This condition misses the DELETE plan used by the added test: metrics("deletedRecords") is absent before the write runs. I ran mvn -pl paimon-spark/paimon-spark-ut -am -Pfast-build -DfailIfNoTests=false -DwildcardSuites=org.apache.paimon.spark.sql.PaimonMetricTest -Dtest=none test, and Paimon Metric: delete fails with NoSuchElementException: key not found: deletedRecords at PaimonMetricTest.scala:188. Since Spark builds the command metrics from supportedCustomMetrics, we need to register PaimonDeletedRecordsMetric for the row-level DELETE write path that actually reaches this test, or ensure operationType is populated before supportedCustomMetrics is queried.