test(mesh): cover Gmsh stub errors without the Gmsh extension

Add a focused test file asserting read_gmsh_msh raises ErrorException and
mesh_from_current_gmsh_model stays undefined until JuliaFEMGmshExt loads.

- Add test_gmsh_stub.jl with two throws checks for the weakdep path
- Include the new file from test/mesh/runtests.jl next to existing mesh tests

test/mesh/runtests.jl

@@ -7,3 +7,4 @@ using Tensors
 
 include("test_refine.jl")
 include("test_mesh_api.jl")
+include("test_gmsh_stub.jl")

test/mesh/test_gmsh_stub.jl

@@ -0,0 +1,10 @@
+# SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+# SPDX-License-Identifier: MIT
+
+using Test
+using JuliaFEM
+
+@testset "Gmsh bridge stubs (extension not loaded)" begin
+    @test_throws ErrorException JuliaFEM.read_gmsh_msh("nonexistent.msh")
+    @test_throws MethodError JuliaFEM.mesh_from_current_gmsh_model()
+end