Parameters for adding a new step to a workflow. step_type_settings must contain
exactly one key naming the step type.

Placement

• Trunk (default): omit parent_step_id and branch to append at the end of the main sequence.
• Inside a split branch: set parent_step_id to the id (integer or public_id) of the split step, and branch to:
  • matched or unmatched for a conditional_split_step (the legacy match/fallback aliases are still accepted)
  • A 0-based variant index string ("0", "1") for a split_test_step
• Specific position: use position (0-based integer) or after_step_id (id of the preceding step).

For split_test_step, the variants array must have exactly 2 entries with integer weights summing to 100.
For conditional_split_step, provide condition.filter_id (integer id or obfuscated RefineFilter public_id).