fix(scheduler): guard missing builder job in sync tasks (#732)

[simongu]

Summary

Fixes a NullPointerException observed when running eval (see #732): reader/retrieval sync tasks dereference a BuilderJob that no longer exists, causing a bare NPE at execution time.

• RetrievalSyncTask — null-check builderJob at entry and retrieval inside the loop (both could be null).
• KagReaderSyncTask — null-check builderJob before readSource, so a null job is never passed down.
• common/util CommonUtils#getScannerReaderConfig / getKagBuilderInput — guard job == null (defense in depth for the read bridge path).
• BuilderController#delete — governance fix: when a BuilderJob is deleted, also tear down its linked SchedulerJob (schedulerService.deleteJob(taskId)), so its DAG instances do not keep running sync tasks against a BuilderJob that no longer exists.

Files changed

• server/core/.../task/sync/builder/RetrievalSyncTask.java
• server/core/.../task/sync/builder/KagReaderSyncTask.java
• common/util/.../CommonUtils.java
• server/api/http-server/.../openapi/BuilderController.java

Test plan

• Local Maven build not possible in the authoring sandbox (no JDK/Maven). Please run:
  mvn -pl common/util,server/core/scheduler/service -am compile
• Runtime repro: run eval that triggers reader/retrieval after the BuilderJob has been removed.

Closes #732