Reject tables inserted into inline tables

[gaoflow]

Summary

• Reject Table values appended or assigned to an InlineTable instead of serializing invalid TOML
• Add a regression test for the public append() and item assignment paths
• Update the unreleased changelog

Closes #531

Tests

• uv run --no-project --with pytest --with pyyaml python -m pytest tests/test_items.py::test_append_table_to_inline_table_raises -q
• uv run --no-project --with pytest --with pyyaml python -m pytest tests/test_items.py tests/test_api.py -q
• uv run --no-project --with pytest --with pyyaml python -m pytest -q
• uv run --no-project --with ruff ruff check tomlkit tests --exclude tests/toml-test
• uv run --no-project --with ruff ruff format --check tomlkit tests --exclude tests/toml-test
• git diff --check

Notes: full ruff check . / ruff format --check . include the initialized tests/toml-test submodule and fail on its upstream gen.py. mypy tomlkit tests --exclude tests/toml-test currently reports existing strict typing errors on master (for example source.py generic annotations and numeric dunder return types in items.py).

AI assistance was used under my direction.