fix(meshing): clear dm_plex_gmsh options after each import (spacedim leak)

[lmoresi]

Summary

A pre-existing core-meshing bug: the dm_plex_gmsh_* options are global,
import-time scratch on the PETSc options database, but they were not cleared
after a gmsh import. A manifold-surface generator sets
dm_plex_gmsh_spacedim = 2 and, if it raises before cleanup (e.g.
SegmentedSphericalSurface2D errors in set_periodicity after the import),
that option leaks — and the next gmsh import (say a SphericalShell) is read
with spacedim=2, producing a 2-D mesh in a 3-D cdim.

Symptom (found via cross-test pollution): after a 2-D manifold generation, a
fresh SphericalShell loads as 2-D and crashes with
cannot reshape array of size 856 into shape (3) — and the corrupt 2-D mesh is
even written to the shell's on-disk cache, so it persists.

Fix

• A (root): clear the whole dm_plex_gmsh_* namespace in a finally after
  every import, centrally in _from_gmsh. These options are meaningful only for
  the single import that follows, so a value set for one import can no longer
  leak into the next — on success or failure.
• B (defence-in-depth): in the Mesh constructor, validate the DM
  coordinate array is divisible by cdim before reshaping; if not, raise a
  clear, actionable error (stale/corrupt cache → delete .meshes/*.msh(.h5))
  instead of an opaque numpy reshape failure.

Validation

Verified on the integration branch where it was found: a failed manifold
construct followed by a SphericalShell now yields a correct 3-D mesh; full
level_1 + tier_a is green; no false positives on valid meshes.

Cherry-picked from validated work on the manifold/extract_surface integration
branch (PR #237), where it gates the surface-extraction tests' CI.

Underworld development team with AI support from Claude Code

[Copilot]

Pull request overview

Fixes a global PETSc options leak during Gmsh mesh import (dm_plex_gmsh_*) that could silently corrupt subsequent imports (notably spacedim leaking from manifold/surface generators into later 3D meshes), and adds a clearer failure mode when cached meshes have inconsistent coordinate data.

Changes:

• Clear dm_plex_gmsh_* PETSc options in a finally block after every _from_gmsh() import to prevent cross-import option leakage.
• Add a coordinate-array size vs cdim validation in Mesh.__init__ to raise an actionable error instead of a NumPy reshape exception.

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