test: add round-trip, wrapper, workflow mutator, and SetLayout tests

- 33 round-trip and wrapper tests for convert/unconvert layer covering
  all forward conversions, reverse (unconvert) paths, nil handling,
  and error passthrough
- 48+ workflow mutator tests: InsertAfterActivity, ReplaceActivity,
  InsertOutcome, DropOutcome, InsertPath, DropPath, InsertBranch,
  DropBranch, InsertBoundaryEvent, SetActivityProperty, SetPropertyWithEntity
- 6 SetLayout tests: basic layout change, explicit param mappings,
  same-layout no-op, snippet rejection, missing FormCall, empty Form
- Widget engine test updates for applyChildSlots operation validation

Author: retran