fix: use valid JobStatus for batch cancellation

[chubes4]

Summary

• Bug: PipelineBatchScheduler::processChunk() was calling complete_job() with 'cancelled' as the status, but 'cancelled' is not in JobStatus::FINAL_STATUSES. The DB layer rejected the update silently, leaving the parent job stuck in 'processing'.
• Fix: Uses JobStatus::failed('batch cancelled')->toString() which produces a valid compound status 'failed - batch cancelled'.
• Test fix: Adds set_up_before_class() to PipelineBatchSchedulerTest to run datamachine_activate_for_site() — the activation hook doesn't fire in the test context, so plugin tables must be created explicitly.

Testing

All 10 PipelineBatchSchedulerTest assertions pass (53 total assertions):

Pipeline Batch Scheduler (DataMachine\Tests\Unit\Engine\PipelineBatchScheduler)
 ✔ Fanout creates batch metadata on parent
 ✔ Fanout stores transient
 ✔ Process chunk creates child jobs
 ✔ Process chunk respects cancellation
 ✔ Child labels use packet titles
 ✔ On child complete marks parent done when all children finish
 ✔ On child complete marks parent failed when all children fail
 ✔ Child jobs receive per item engine data
 ✔ Child jobs work without engine data key
 ✔ On child complete ignores non batch parents

Run with: homeboy test data-machine --skip-lint --path=<workspace> --setting database_type=mysql --setting mysql_database=extrachill --setting mysql_user=extrachill --setting mysql_password=<pw> -- --filter PipelineBatchSchedulerTest