Honor custom parsing failure comments

[sapunyangkut]

Summary

• Honor the active BibtexFormat.parsing_failed_comment template when writing failed blocks.
• Strengthen the existing regression test for both the default and a custom template.

Fixes #530

Validation

Preserved exact-SHA evidence for this unchanged upstream base and identical patch:

• pytest tests/test_writer.py::test_write_failed_block: 1 passed
• pytest tests/test_writer.py: 25 passed
• configured pre-commit hooks on both changed files: passed

The current no-install checkout lacks pylatexenc, so its fresh focused rerun stopped during collection. No dependencies or dependency files were changed.

AI assistance

This Draft PR was prepared with OpenAI Codex. Automated root-cause analysis, patching, and validation evidence were used; no human self-review is claimed. It remains a draft for maintainer and user review.

[MiWeiss]

This Draft PR was prepared with OpenAI Codex. Automated root-cause analysis, patching, and validation evidence were used; no human self-review is claimed. It remains a draft for maintainer and user review.

@sapunyangkut Please review and motivate your PRs in the future; i tend to close ai-only PRs. Exception here, as it is a trivial fix :-)