tests: cover Vulkan mesh shader opt-in wiring

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Risky behavior now covered

• Optional VK_NV_mesh_shader device-extension enablement stays explicitly opt-in through r_vk_meshShaderNV.
• The extension is appended only when the driver reports support, the cvar is enabled, and the fixed extension list has capacity.
• Device creation keeps the required VkPhysicalDeviceMeshShaderFeaturesNV pNext chain with task shaders disabled and mesh shaders enabled.

Test files added/updated

• Added tests/scripts/test_vulkan_mesh_shader_opt_in.sh.
• Registered test_vulkan_mesh_shader_opt_in in CMakeLists.txt with unit;scripts;validation;renderer labels.

Why this reduces regression risk

• Mesh shader extension wiring affects Vulkan device creation, so an accidental default-on change or missing feature chain can break startup on supported hardware.
• The test is deterministic and source-level, avoiding GPU/hardware dependencies while still guarding the high-risk invariants.

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