[Caideyipi]

Description

On dev/1.3, rollback can close a Load task's writers while its RPC work is still running. Track active work, including nested slice assembly and writing, so rollback rejects further work while cleanup is pending and releases resources after the last active call finishes.

This also preserves the other improvements from #18772:

• Grow the sliced Load RPC buffer as bytes arrive instead of preallocating the peer-declared original body size.
• Exclude the active-load failure directory from scanning when it is nested beneath a listening directory.

The cleanup changes target the legacy global Load manager on dev/1.3. #18698 replaced that lifecycle on master with per-DataRegion consensus commands and consensus-aware staging retention, so #18772 is being replaced by this PR for the legacy implementation. Adapt the scanner regression test to the dev/1.3 loader API and cover rollback while nested work remains active.

Validation

• mvn -o -pl iotdb-core/datanode spotless:apply

• mvn -o -pl iotdb-core/datanode -am "-Dtest=LoadTsFilePieceNodeAssemblerTest,ActiveLoadDirScannerTest,LoadTsFileManagerTest" "-Dsurefire.failIfNoSpecifiedTests=false" test — 13 tests passed, no failures or errors.

• mvn -o test-compile -DskipTests — full reactor passed (44 modules).

• mvn -o test-compile -P with-zh-locale -DskipTests — full reactor passed (44 modules).

• Self-reviewed concurrent task admission, rollback and cleanup.

• Added or adapted regression tests for all three changes.