NullPointerException in ParallelMultiInstanceBehavior.leave() when completing a user task (execution is null in setLoopVariable)

### Describe the bug
When completing a **parallel multi-instance (会签) user task**, the engine throws a `NullPointerException` because `execution` is null inside `MultiInstanceActivityBehavior.setLoopVariable()`, called from `ParallelMultiInstanceBehavior.leave()`.
### Stack trace
java.lang.NullPointerException: Cannot invoke "org.flowable.engine.delegate.DelegateExecution.setVariableLocal(String, Object)" because "execution" is null at org.flowable.engine.impl.bpmn.behavior.MultiInstanceActivityBehavior.setLoopVariable(MultiInstanceActivityBehavior.java:579) at org.flowable.engine.impl.bpmn.behavior.ParallelMultiInstanceBehavior.leave(ParallelMultiInstanceBehavior.java:296) at org.flowable.engine.impl.bpmn.behavior.ParallelMultiInstanceBehavior.internalLeave(ParallelMultiInstanceBehavior.java:233) at org.flowable.engine.impl.bpmn.behavior.ParallelMultiInstanceBehavior.leave(ParallelMultiInstanceBehavior.java:174) at org.flowable.engine.impl.bpmn.behavior.AbstractBpmnActivityBehavior.leave(AbstractBpmnActivityBehavior.java:60) at org.flowable.engine.impl.bpmn.behavior.UserTaskActivityBehavior.trigger(UserTaskActivityBehavior.java:340) at org.flowable.engine.impl.bpmn.behavior.MultiInstanceActivityBehavior.trigger(MultiInstanceActivityBehavior.java:325) ... at org.flowable.engine.impl.TaskServiceImpl.complete(TaskServiceImpl.java:236)

### Environment
- **Flowable version:** 7.2.0
- **Integration:** `flowable-spring-boot-starter` 7.2.0
- **Java:** 17
- **Database:** PostgreSQL (if relevant: we use default Flowable schema; column `IS_MI_ROOT_` / `is_mi_root_` exists on `ACT_RU_EXECUTION`)
### To Reproduce
1. Deploy a process with a **parallel multi-instance user task** (e.g. 2 instances, assignee list from a variable).
2. Start the process so that the MI activity creates 2 child executions and 2 tasks.
3. Complete **one** of the user tasks (e.g. via `taskService.complete(taskId)` or REST).
4. The NPE occurs during that `complete()` call when the engine runs `ParallelMultiInstanceBehavior.leave()` and then `setLoopVariable(execution, ...)` with a null `execution`.
### Expected behavior
The first completed instance should leave the activity without error; the multi-instance root execution should be found and used for `setLoopVariable`, and the second instance should remain active.
### Actual behavior
`getMultiInstanceRootExecution(execution)` (or equivalent) appears to return null, so the execution passed to `setLoopVariable` is null and an NPE is thrown.
### Additional context
- We have two child executions under the MI scope execution; only one task is completed when the error happens.
- The MI scope execution in the DB has multi-instance variables (`nrOfInstances`, `nrOfCompletedInstances`, `nrOfActiveInstances`). We did **not** modify Flowable engine code; we only call `taskService.complete(taskId)` (and before that we update task assignee/category and set process variables on the root execution).
- We tried (without resolving the issue):
- Overriding `Execution.xml` to map the `is_mi_root_` column (PostgreSQL lowercase) so that `isMultiInstanceRoot()` is set correctly when loading executions.
- Moving `updateBusinessStatus()` and `setVariables()` to **after** `complete()` so that no extra writes touch executions/variables before the engine’s leave logic runs.
- We are happy to provide a minimal BPMN or database snapshot if needed for reproduction.