tests: cover Vulkan mesh shader opt-in

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Risky behavior now covered

• Optional VK_NV_mesh_shader device enablement remains explicitly opt-in, default-off, range-limited, latched, and capacity-guarded.
• Device feature pNext chaining only occurs after the extension is selected, keeps taskShader disabled, and enables only meshShader for the future mesh path.

Test files added/updated

• Added tests/scripts/test_vulkan_mesh_shader_opt_in.sh
• Registered test_vulkan_mesh_shader_opt_in in CMakeLists.txt

Why this reduces regression risk

• The mesh shader path sits at Vulkan device creation, so accidental default enablement or incorrect feature chaining can break renderer startup on affected drivers.
• The test is deterministic and source-invariant based, so it runs in headless CI without Vulkan hardware while still guarding the high-risk contract.

Validation

• tests/scripts/test_vulkan_mesh_shader_opt_in.sh
• cmake -S . -B build-coverage-tests -DBUILD_TESTING=ON -DBUILD_UNIT_TESTS=OFF
• ctest --test-dir build-coverage-tests -R test_vulkan_mesh_shader_opt_in --output-on-failure