fix: close Array.add_line output before the closing bracket

[Neallin-917]

Summary

• preserve a trailing line break for content added with Array.add_line()
• avoid emitting a comment immediately before the closing ], which produces invalid TOML
• add a regression test that parses the generated output

Fixes #580

Validation

• poetry run pytest tests/test_items.py -k array_add_line -q
• poetry run pytest -q tests (1052 passed)
• uvx ruff@0.15.21 check tomlkit/items.py tests/test_items.py
• poetry run python -m compileall -q tomlkit tests/test_items.py

poetry run mypy tomlkit/items.py tests/test_items.py still reports the pre-existing tomlkit/items.py:2172 annotation error.

Disclosure

AI assistance was used while preparing this contribution; I reviewed the change and ran the validation above.