fix: quote POSIX world paths at launch entry points

[yeseorizi]

Problem

The world and robot launch entry points pass an unquoted world filename through gz_args to the shell-based Gazebo launcher. An installed package share containing spaces (for example, a native macOS application data directory) is consequently split into multiple arguments, and the selected world cannot load.

Change

• Quote the resolved world filename on POSIX in both posim_world.launch.py and posim_robot.launch.py.
• Preserve existing launch flags, the empty.sdf case, and Windows command construction.
• Add 20 regression cases and register them with ament_cmake_pytest.

Validation

• New tests against the original files: 12 failed / 8 passed; with the fix: 20 passed on both macOS ARM64 and Linux ARM64 Docker (ROS 2 Lyrical).
• Tests execute the generated arguments through /bin/sh and compare the received filename, including spaces, apostrophes, Unicode, and shell metacharacters. They also check Windows command construction and empty-world flags.
• Native colcon build / colcon test for posim_demos only pass; changed-file pre-commit hooks pass.
• The earlier combined executable/world-path integration matrix passed 14/14 actual headless Gazebo launch/exit cases per environment on macOS ARM64 and Linux ARM64 Docker. It used the latest launch-file snapshots with installed engines, not a fresh full-repository build. The robot entry point was checked for Gazebo argument generation, not full robot spawning.

python3 -m pytest -q examples/posim_demos/test/test_world_paths.py

Related boundary: quoting the gz executable itself is handled by ros_gz #980; this PR fixes the separate world filename argument. If both paths contain spaces, both fixes are needed.

Not claimed: Windows execution or fixes, Linux AMD64 validation, or a complete packaged-app/robot-demo acceptance test.

• Focused change on a fork
• Regression tests and local package build/test
• Changed-file pre-commit checks
• Remote CI (to be checked after submission)

AI assistance: OpenAI Codex was used to prepare the patch, tests, and this description; the checks above were executed locally.