central gap between sectors

[MohamedNumair]

The sectors implementation currently doesn't have proper handling of the air gap between the sectors for the FEM results, hence has a non solved region between the sectors

I am opening this issue and assigning to my self to patch later

[MohamedNumair]

A Fix should be cherry-picked from this commit 73c8ceb

The gap issue was in the mesh/entity-identification path, not in the GetDP material definitions. I matched the existing wire-array approach by seeding sector-bundle voids with air markers after cable geometry creation. The new helper derives gap directions from the sector rotations, drops markers into the interstitial sector voids, and registers them with the ambient-air region material so enclosed sector gaps get a physical group instead of remaining unlabeled. I also hardened physical-group assembly so repeated markers hitting the same final surface do not duplicate entity tags.

I added a regression around the sector FEM case and changed it to assert against workspace.space_regions, which is the correct post-mesh state because make_mesh! finalizes Gmsh.

[MohamedNumair]