Backport robust pytest collection hooks to Humble

[Old-Ding]

Summary

• backport the final launch_testing pytest collection hooks from Fix Pytest 8/9 compatibility in hooks #972 and Robust pytest_ignore_collect for multi-version Pytest compatibility #974 to Humble
• use pytest 8/9's module_path hook while preserving the legacy path argument for pytest 7 and older plugins
• avoid eager imports of launch files, non-Python files, and the logging.handlers collision during collection

Fixes #979

Testing

• reproduced the Humble baseline failure with pytest 9.1.1: PluginValidationError because pytest_pycollect_makemodule(path, parent) no longer matches the hookspec
• reproduced the Humble baseline over-collection with pytest 8.4.2
• pytest 7.4.4: 83 launch_testing tests passed; launch test collection passed
• pytest 8.4.2: 83 launch_testing tests passed; launch test collection passed
• pytest 9.1.1: 83 launch_testing tests passed; launch test collection passed
• targeted ament_flake8 on both changed files
• ament_copyright and ament_pep257
• compileall, UTF-8 without BOM, CRLF, added-line 100-column check, and git diff --check

The two changed files exactly match the versions already merged and CI-validated on the lyrical branch in #973.

Did you use Generative AI?

Yes. OpenAI Codex was used to trace the Humble failure, separate the launch_testing backport from unrelated launch_pytest prerequisites, apply the already-merged Lyrical implementation, and run the pytest 7/8/9 regression matrix. The contributor reviewed the final two-file diff and test results.

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