Previously, background_cancel(all=true) only cancelled direct child tasks, leaving grandchildren and deeper nested tasks uncancelled. This caused background agents to continue running even when their parent session was cancelled.
Changes:
- Added getAllDescendantTasks() method to BackgroundTaskManager for recursive task collection
- Updated background_cancel to use getAllDescendantTasks instead of getTasksByParentSession
- Added comprehensive test coverage for nested task cancellation scenarios
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