fix: write file exports with LF newlines

[w3lld1]

Summary

Fixes #325 by writing file exports with explicit LF newlines instead of relying on the platform default text newline translation.

Changes

• Pass newline="\n" when writing exported files to disk.
• Add a regression assertion that generated requirements.txt bytes do not contain CRLF line endings.
• Leave stdout/IO output unchanged.

Verification

• pytest tests/test_exporter.py::test_exporter_can_export_requirements_txt_with_standard_packages -q
• pytest -q
  • 178 passed
• ruff check src tests
• python -m py_compile src/poetry_plugin_export/*.py
• git diff --check

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud