[core] Keep staging trees out of an overwrite that clears a prefix

[sundapeng]

Follow-up to #8861, which did this for the read path. The overwrite path still decides what to delete from the leaf file name alone.

Problem

INSERT OVERWRITE lists the partition it clears with fileIO.listFiles(path, true) and deletes every entry whose name is not '_'/'.'-prefixed. Files another job staged there carry ordinary data file names, so they are deleted and that job then fails to commit its own output. Only the directory above them says they are uncommitted, which is exactly what the read path started using.

The prefix case is worse. INSERT OVERWRITE ... PARTITION (year = '2025') on a table partitioned by year, month clears year=2025, and a job writing the same prefix with a dynamic month stages under it:

year=2025/_temporary/attempt/part.csv
year=2025/_temporary/0/attempt_x/month=12/part-00012.csv

Both go, because every intermediate directory name looks ordinary.

Fix

Delete what listDataFiles returns, and give it the depth the static partition left open — a static partition may name only the leading keys, so the partition directories of the remaining keys sit below the path being cleared.

Being at a partition level does not exempt a directory from the '_'/'.' rule. year=2025/_temporary holds the other job's own month directories, not this table's. One hidden name is table content: the default partition name in the value-only layout, where a partition directory is the bare value — the exemption partition discovery already makes. In the key=value layout a partition directory is always month=..., so nothing hidden is descended at all.

FormatTableScan.isDataFileName has no caller left after this and is removed in the second commit; its three name-convention tests move next to the rule they are about, PartitionPathUtils.isHiddenName.

Tests

• testOverwritingAPrefixKeepsStagingTreesSittingAtAPartitionLevel — a staging root standing where the dynamic month directories do, in three shapes, while the stale committed file still goes.
• testOverwritingAPrefixClearsTheDefaultPartitionDirectory — the value-only default partition is cleared although its name starts with _, while a _temporary next to it survives. That pair is what separates "exempt this one name" from "exempt anything hidden here".
• testOverwriteKeepsFilesOfConcurrentWritersStagingTrees — a magic-committer tree inside a fully specified partition.

mutation	result
descend any hidden directory at a partition level	2 tests fail
pass depth 0 instead of partitionKeys - staticPartitions	1 test fails

135 tests pass across FormatTableScanTest, FormatTableCommitTest, CatalogManagedPartitionScanTest, PartitionPathUtilsTest, FormatTableWriteTest, FormatReadBuilderTest, FormatTableCompatibilityTest and FormatDataSplitTest; spotless and checkstyle clean.

Note on _temporary

testOverwriteKeepsFilesOfConcurrentWritersStagingTrees covers the magic-committer tree only. A _temporary in the same partition is emptied by this writer's own RenamingTwoPhaseOutputStream.clean(), which still removes that shared directory recursively; #8900 fixes that separately. Once it lands, the _temporary shape belongs in this test too.