Fix validation of nested out-of-order tables

[satriadhikara]

Summary

• validate each raw out-of-order table fragment instead of materializing an
  OutOfOrderTableProxy during concrete/super-table validation
• add a regression test for a valid nested out-of-order table separated by an
  unrelated array-of-tables

The proxy was incorrectly treated as a non-table, causing valid TOML to raise
KeyAlreadyPresent. Validating the underlying Table fragments preserves the
existing collision checks without special-casing the proxy.

Closes #571.

Checks

• python -m pytest -q — 1052 passed
• ruff check tomlkit tests --exclude tests/toml-test
• ruff format --check tomlkit tests --exclude tests/toml-test

The repository's strict mypy run still reports five pre-existing errors in
untouched source.py, items.py, and api.py.

Agent Drafting Metadata

• Agent: OpenAI Codex
• Model: GPT-5.6 Sol
• Notes: Codex reproduced and minimized the regression, drafted the fix and
  tests, and prepared the issue and PR text.