`CoalescePartitionsExec` delays cancellation of child operators

[Samyak2]

Describe the bug

• CoalescePartitionsExec tasks hold on to an Arc ref of the input plan (ref)
  • It doesn't actually need it after input.execute(..) except for printing the plan in debug logs
• RepartitionExec has some state in the plan itself
  • One such state holds on to Arc refs of Arc<Vec<SpawnedTask<()>>> (ref)
  • Which means that the tasks are only cancelled once all Arc refs to the plan are dropped
  • So each layer of CoalescePartitionsExec-RepartitionExec delays cancellation of the query

To Reproduce

I have a reproducer here: Samyak2#1 (warning: mostly LLM-generated, but I have verified that it actually checks the correct thing)

Relevant parts of the output:

repartition_task_group=0 input_partition=0 kind=pull_from_input drop_elapsed_ms=68
repartition_task_group=1 input_partition=0 kind=pull_from_input drop_elapsed_ms=80
repartition_task_group=1 input_partition=1 kind=pull_from_input drop_elapsed_ms=85
output_partitions=32 input_rows_per_partition=1024000 all_repartition_operator_drop_elapsed_ms=80
all_repartition_task_drop_elapsed_ms=85
all_observed_drop_elapsed_ms=85

The cancellation is delayed by ~80ms due to CoalescePartitionsExec

Expected behavior

CoalescePartitionsExec should drop child plan early

Additional context

I have a fix for this. Will raise a PR soon.