Cliff impacts don't successfully complete for unclear reasons

Bug

Cliff impacts (the marginal-cliff analysis view) don't successfully complete for some policies, for unclear reasons — the request returns a completed-but-empty payload ("result": null with "status": "ok") instead of the expected result structure. When this happens, other output views for the same policy (e.g., the policy breakdown) can also fail to display anything.

Reproduction pattern

A completed network call for a cliff-impact computation returns something like:

{
  "average_time": 6870.44,
  "message": null,
  "result": null,
  "status": "ok"
}

instead of a populated result containing the cliff-impact structure.

Scope

Root cause was unclear in the original report and needs fresh investigation against v2's simulation/report pipeline — worth tracking since a silent result: null on status: ok masks a real backend failure as success.

Migrated from policyengine-app#2626.

Start by reproducing the cliff-impact request in the v2 simulation/report pipeline and inspect the completed response for result: null with status: ok. Compare it with a policy whose result is populated and check whether the policy breakdown is affected too. Done means the pipeline's failure point is identified and the issue no longer silently reports an empty successful result.