Isolate and strengthen field-sorting tests

[claell]

Summary

• Isolate in-place field-sorting cases with fresh entry fixtures.
• Verify stable ordering for fields omitted from a custom order.
• Exercise custom sorting through the public writer without source mutation.
• Reject ambiguous custom orders, including case-only duplicates.

Closes #577.

Validation

• pytest -p no:cacheprovider -W error tests/middleware_tests/test_sorting_entry_fields.py: 8 passed.
• Complete upstream test suite: 2,579 passed, 12 skipped, with the four existing deprecation warnings in tests/test_entrypoint.py.
• Complete pre-commit hook suite: passed.

Review note

This PR changes only tests. In particular, it replaces an execution-order-dependent expected value with the stable ordering produced from a fresh input. Maintainer review should confirm that unspecified fields retaining their original relative order is the intended contract.

AI assistance

This pull request was prepared with ChatGPT Codex using GPT-5.6 Sol with high reasoning effort. Codex assisted with test isolation, behavior analysis, branch preparation, and validation. Automated validation is not a substitute for maintainer review.

[MiWeiss]

Hi @claell — same note across all of your July 16 issues and PRs (#567–#597), apologies for the repetition.

Closing this. Passing CI isn't evidence the underlying claim is real — it just means generated tests match generated code. The batch's timing, structure, and forward-referenced numbers point to this being AI-generated rather than manually found and checked, and reviewing it properly would take more effort than a targeted pass over the parser myself.

If this fixes something you actually hit: open a small, human-verified PR against a reopened issue with a real repro, and I'll review it in good faith. Any nontrivial design or API choice needs to be discussed and agreed first, not decided inside a PR. Please disclose and verify AI-assisted work before submitting going forward.