This commit improves the workflow step activation process in the assistant panel:
- Introduces a `StepAssist::editor_was_open` struct to track if an editor was already open
- Updates `cancel_workflow_step_if_idle` to respect pre-existing editors
- Simplifies the `ActiveWorkflowStep` concept to just track the step range
These changes ensure that we don't inadvertently close buffers that were
already open when activating or deactivating workflow steps, providing a
more stable and predictable editing experience.
a241c06672