[codex] support path-like image paths

[Sean-Kenneth-Doherty]

Summary

• accept os.PathLike image paths in the image loader used by Document.add_picture() and Run.add_picture()
• update public image-path annotations and docs to mention path-like objects
• add regression coverage for Image.from_file(Path(...))

Fixes #1544

Validation

• uv run --with pytest --with ruff --with-editable . python -m pytest tests/image/test_image.py tests/test_document.py tests/text/test_run.py -q
• uv run --with pytest --with ruff --with-editable . python -m ruff check src/docx/image/image.py src/docx/document.py src/docx/text/run.py src/docx/parts/story.py src/docx/package.py tests/image/test_image.py
• uv run --with-editable . python - <<'PY' ... Document().add_picture(Path('tests/test_files/python-icon.png')) ... PY
• git diff --check

Note: a targeted pyright run is not included as validation because the current project reports pre-existing strict-type errors in these files/tests before this change.

AI assistance: prepared with OpenAI Codex.