#722 Refactor wait_for_parallel_branches to use JoinMode for consistency

Currently wait_for_parallel_branches uses ad-hoc fail_on_error=True boolean while JoinOperator has proper JoinMode enum (ALL_SUCCESS, ALL_OF, ANY_OF, ONE_SUCCESS). This creates API inconsistency. Current: wait_for_parallel_branches(fail_on_error=True) Proposed: wait_for_parallel_branches(join_mode=JoinMode.ALL_SUCCESS) Files: engine/tools/parallel_join.py, engine/platform_core/workflows/platform_bootstrap.py