fix: keep newline after dotted inline table

[gaoflow]

Summary

• preserve the trailing newline from a parsed dotted-key super table when appending a following sibling key
• add regression coverage for adding a key after [x]\na.b = {} with and without a final newline

Closes #440.

Tests

• uv run --no-project --with pytest --with pyyaml python -m pytest tests/test_toml_document.py::test_add_key_after_dotted_inline_table_without_ending_newline -q
• uv run --no-project --with pytest --with pyyaml python -m pytest tests/test_toml_document.py -q
• uv run --no-project --with pytest --with pyyaml python -m pytest -q (1024 passed)
• uv run --no-project --with ruff ruff check tomlkit/container.py tests/test_toml_document.py
• uv run --no-project --with ruff ruff format --check tomlkit/container.py tests/test_toml_document.py
• git diff --check

AI assistance was used under my direction.