Fix false disk-full read-only during concurrent snapshot directory de…

[hongzhi-gao]

Description

Problem

During datanode removal, multiple DataRegions may concurrently migrate snapshots to the same target node. All snapshot staging directories share one root (data/datanode/data/snapshot/). While one region is receiving snapshots, getNextFolder() walks this shared root to compute occupied space; if another region deletes its staging subdirectory at the same time, Files.walk throws NoSuchFileException. This was misinterpreted as "folder full" → node switched to read-only (DISK_FULL) → snapshot transmission failed → region migration / datanode removal blocked.

Fix

1. JVMCommonUtils.getOccupiedSpace: Replace Files.walk with walkFileTree; return CONTINUE in visitFileFailed to ignore entries deleted during traversal.
2. MinFolderOccupiedSpaceFirstStrategy: On refresh failure, keep the previous cached occupied space instead of setting Long.MAX_VALUE.
3. FolderManager: Only switch to read-only when no healthy folder has available disk space (hasSpace()), not merely when folder selection fails.

Tests

• JVMCommonUtilsTest.getOccupiedSpaceIgnoresConcurrentlyDeletedEntries
• MinFolderOccupiedSpaceFirstStrategyRealFsTest.getNextFolderDoesNotEnterReadOnlyWhenDiskHasSpace

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This PR has:

• been self-reviewed.
  • concurrent read
  • concurrent write
  • concurrent read and write
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods.
• added or updated version, license, or notice information
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage.
• added integration tests.
• been tested in a test IoTDB cluster.

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Key changed/added classes (or packages if there are too many classes) in this PR

• org.apache.iotdb.commons.utils.JVMCommonUtils
• org.apache.iotdb.commons.disk.strategy.MinFolderOccupiedSpaceFirstStrategy
• org.apache.iotdb.commons.disk.FolderManager
• org.apache.iotdb.commons.i18n.UtilMessages