[KennyMcSimpson]

Summary

Appending to x = [1\n# note\n] produces invalid TOML: the new value is written after the comment without a separator after 1. This affects append, extend, and insertion beyond the end when the final value has no trailing comma.

Use the index map to find the preceding actual value when adding its missing comma. Keep the existing formatting group for indentation, so standalone comments and array layout are preserved. Add regressions for all three operations, consecutive comments and existing-comma controls, plus a changelog entry. This is separate from #666, which only optimizes index rebuilding.

Validation: the added regressions produce six failures and six passing controls on unchanged master. The complete suite passes on Python 3.13 (1,215 tests) and Python 3.10 (1,214 passed, one skipped), including the pinned toml-test corpus. All pre-commit hooks pass. Independent checks with stdlib tomllib also pass. Strict mypy reports the same six pre-existing errors on both unchanged source and this patch; none are introduced in the changed lines.

Agent Drafting Metadata

• Agent: OpenAI Codex
• Model: GPT-6
• Notes: The problem was reproduced, fixed and tested by the coding agent, with a separate agent reviewing the change. No independent human review is claimed.