[To dev/1.3] Load: preserve pending tablets when parser fails (#18487)

[Caideyipi]

Problem

When tree-model Load falls back from scan parsing to query parsing, a parser exception can occur after earlier Tablets have been converted but before the pending batch reaches its normal flush boundary. The exception path released the conversion context and discarded that batch.

Fix

Flush pending Tablets before returning a normal parser error. Temporary-unavailable results retain the pending batch and original parser status for the next managed-task retry; memory-pressure and interruption exceptions keep their existing retry behavior.

Tests

• mvn spotless:apply -pl iotdb-core/datanode (passed)
• Checkstyle: 0 violations
• mvn test -pl iotdb-core/datanode -Dtest=LoadTreeStatementDataTypeConvertExecutionVisitorTest -DfailIfNoTests=false (blocked by pre-existing DataNode compilation errors unrelated to this change)