Fail closed in DAG postprocessing

[oshaughnessy-junior]

Summary

• resolve ILE and NR postprocessing helpers relative to the installed wrapper, with a guarded PATH fallback
• propagate cleaner, sort, and relabel pipeline failures instead of accepting empty products
• require nonempty composite and indexed outputs before packaging success
• add sanitized-PATH regression coverage for missing helpers, sibling resolution, helper failures, and empty outputs

Production motivation

Calibration reruns exposed a fail-open path: util_ILEdagPostprocess.sh was invoked by absolute path, but its bare util_CleanILE.py call was absent from the worker PATH. The wrapper still returned zero and left an empty consolidated_N.composite, so failure appeared only in a later DAG POST check.

This patch does not hardcode any production environment path or alter DAG science configuration.

Testing

• bash -n on both modified wrappers
• pytest -q MonteCarloMarginalizeCode/Code/test/test_dag_postprocess_fail_closed.py (6 passed)

This is the O4c half of a coordinated O4c/O4d fix. Paired O4d PR: oshaughnessy-junior#373