[FEATURE]: when a workflow is removed, not all of its children are removed.

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## Describe the Feature Request
_When a workflow is removed, not all of its children are removed._

## Describe Preferred Solution
_You could include in the removal of a workflow the search for children via parent_workflow_id to then be able to remove them together with the connected tasks._

Start by tracing the workflow-removal entry point and the parent_workflow_id lookup described in the issue. Inspect how child workflows and their connected tasks are currently deleted, then add coverage showing that removing a parent also removes all children and connected tasks.

Written by the indexing model from the issue text.