[spark] Do not display an unknown partition statistic as a number

[sundapeng]

Summary

A negative partition statistic means that the field was not reported; PartitionStatistics.isKnown is the contract introduced by #9121. Spark's partition-detail commands still displayed negative sentinels directly, and SHOW TABLE EXTENDED ... PARTITION treated an absent row count or byte size as an exact zero.

This change keeps unknown values out of numeric display output. It is display-only: it does not write statistics back to a catalog or change query planning.

Changes

• DESCRIBE ... PARTITION omits negative statistics from partition parameters, renders an unknown creation time as UNKNOWN, and creates the temporary display CatalogStatistics only when byte size is known. Row count is handled independently.
• SHOW TABLE EXTENDED ... PARTITION renders missing, nonnumeric, or negative row counts and byte sizes as UNKNOWN. Recognized negative Paimon statistic parameters are omitted; unrelated and nonnumeric metadata is preserved.
• Empty partition-metadata maps keep their existing behavior. This PR does not change metadata producers or the PaimonFormatTable empty-metadata contract.

Testing

• RED on apache/master (87eb04451d): UnreportedPartitionStatisticsTest had 2 passing and 4 failing cases, including 0 rows, 0 bytes for absent keys and -1 in mixed-known output.
• GREEN with this change: UnreportedPartitionStatisticsTest 6/6.
• Spark 3 catalog-managed partition regression suites 29/29.
• Spark 3.5 DescribeTableTest 4/4.
• Spark 3 / Scala 2.12 clean reactor compilation and targeted suite 6/6 with JDK 17.
• Spark 3 / Scala 2.13 clean reactor compilation and targeted suite 6/6 with JDK 17.
• Spark 4 / Scala 2.13 clean reactor compilation and targeted suite 6/6 with JDK 17.
• Spotless check for paimon-spark-common and paimon-spark-ut.
• GitHub Actions: 12/12 checks passed on 178071116b.

Notes

The regression suite uses a native Paimon table plus catalog/table wrappers to inject negative, absent, and partially reported metadata. The stock in-repo REST catalog currently materializes zeros, so it cannot naturally produce these contract inputs. The suite does not claim to exercise a real PaimonFormatTable route.

[JingsongLi]

+1