Fix dotted test filenames in importlib mode

[marko1olo]

Closes #14514.

Summary:

• Sanitizes dotted path components in compute_module_name() so importlib mode derives valid module names for files like foo.test.py.
• Adds package-resolution and import-path regression coverage for dotted test filenames.
• Adds the required changelog fragment and AUTHORS entry.

Tests:

• PYTHONPATH=src python -m pytest testing/test_pathlib.py -q
• python -m py_compile src\_pytest\pathlib.py testing\test_pathlib.py
• git diff --check HEAD~1 HEAD

Checklist:

• Include new tests or update existing tests when applicable.
• Add text like closes #XYZW to the PR description and/or commits.
• If AI agents were used, they are credited in Co-authored-by commit trailers.
• Create a new changelog file in the changelog directory.
• Add yourself to AUTHORS in alphabetical order.

AI assistance disclosure:

• OpenAI Codex assisted with this change; the commit includes Co-authored-by: OpenAI Codex <codex@openai.com>.