This commit addresses a critical bug where subtasks would disappear when
updating the parent task (e.g., assigning tags). The issue had multiple
potential causes:
1. Backend vulnerability: The updateSubtasks() function would delete all
subtasks if an empty array was sent, treating it as "delete everything
not in this list"
2. Frontend state management: After reloading a task, subtasks weren't
being preserved if the backend response didn't include them
3. Unclear error messages: "Invalid parent task" errors didn't provide
enough context for debugging
Changes:
- Added defensive logging in updateSubtasks() to warn when all subtasks
are being deleted with an empty array
- Enhanced validateParentTaskAccess() error messages to provide detailed
diagnostics (task not found vs. permission issues vs. wrong user)
- Updated handleTagsUpdate() in TaskDetails to explicitly preserve
subtasks when reloading task after tag updates
This fix is defensive in nature and adds better observability for
diagnosing similar issues in the future.
Fixes issue reported by user where subtasks disappeared after assigning
tags to parent task, and "Invalid parent task" errors occurred when
trying to update the orphaned subtasks.
