Preserve whitespace when replacing super tables

[gaoflow]

Fixes #352.

Summary

• treat nested table whitespace as table-ending whitespace when deciding whether to add cosmetic blank lines
• add a regression test for reassigning a parsed super table without changing output

Verification

• uv run --no-project --with pytest --with pyyaml python -m pytest tests/test_toml_document.py::test_replace_super_table_preserves_whitespace -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
• 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
• python -m compileall tomlkit/container.py tests/test_toml_document.py
• git diff --check