test(postprocessing): check signed magnitudes in 1-D faceflow cases

[TameTheGame]

The existing one-dimensional get_structured_faceflows cases check whether the active-direction array contains nonzero values. Add an independent physical reference: unit conductivity and cell dimensions with prescribed heads 11, 12 and 13 imply face flows [-1, -1, 0] toward increasing column, row or layer number. Check the absent terminal face is exactly zero.

This adds ten lines and preserves all seven existing parameterizations, model inputs, simulation calls and dependencies. It is a test improvement; no existing implementation defect is claimed.

Validation on base 18dc192cac9ed731dc84f40bc898e0f4d9f87e20:

• All seven targeted cases passed on Windows with zero skips using Python 3.12.13, NumPy 2.3.5, pytest 8.4.2 and the official MODFLOW 6.8.0 Windows release asset, which reports 6.8.0+8680167.dirty.
• Observed execution confirmed both new assertions ran for all three one-dimensional cases; active-face outputs were exactly [-1, -1, 0].
• Ruff 0.16.6 lint and format checks passed for the changed file.

The full FloPy test suite and project-wide release checks were not run locally. The upstream base has successful workflows, but those results do not validate this patch. I am opening this as a draft pending CI and maintainer review. Please let me know whether the targeted local checks plus CI on this commit are sufficient for this assertion-only change, or whether you require additional local validation before this leaves draft status.

Related PR #2746 adds broader conversion/round-trip coverage and leaves this existing assertion block unchanged at the head inspected. This patch is limited to the existing physical 1-D cases.

AI assistance: OpenAI Codex using GPT-6 Astra assisted with the assessment, patch and local validation.