[To dev/1.3] Fix schema region snapshot memory accounting (#18750)

[Caideyipi]

Backport of #18750 to dev/1.3.

Changes

• Reserve the schema Ratis log appender buffer only once across repeated region initialization.
• Release the direct buffer accounting only when it was reserved.
• Preserve both PBTree snapshot operations and report success only when both complete successfully.
• Add coverage for repeated failed snapshot loads and direct buffer accounting cleanup.

Test

• mvn -o test -pl iotdb-core/datanode -Dtest=SchemaRegionManagementTest -DfailIfNoTests=false -DskipITs
• Tests run: 20, failures: 0, errors: 0, skipped: 4

Original commit: d74b274