Fix ty type-check errors: missing imports and unresolved names

- gizmos.py: TYPE_CHECKING-guard `import bmesh` for the string-literal
  annotation in build_schematic_mesh; suppress the still-unresolved
  gizmo_textures import in TexturedQuadGizmoMixin (WIP dependency, not dead
  code).
- model/__init__.py: register the `decorator` submodule, which unregister()
  already calls (would have raised NameError on addon disable).
- mep.py / tool/model.py: add explicit imports for bonsai.core.geometry and
  bonsai.core.model, previously only reachable by accident of import order.
- Test files: add explicit ifcopenshell.api.pset / ifcopenshell.util.element
  submodule imports used but not imported.

(cherry picked from commit 4fb8af2278)

src/bonsai/test/tool/test_model.py

@@ -23,6 +23,7 @@ import bpy
 import ifcopenshell
 import ifcopenshell.api.geometry
 import ifcopenshell.api.material
+import ifcopenshell.api.pset
 import ifcopenshell.api.root
 import ifcopenshell.api.style
 import ifcopenshell.api.type