Preserve iterable middleware arguments

[claell]

Summary

• Materialize one-shot append_middleware and prepend_middleware iterables
  before checking their types.
• Ensure those middleware objects still execute after duplicate detection.
• Correct the unparse-stack duplicate warning to name the unparse parameters.

Fixes #531.

Validation

• Focused iterator regressions under warnings-as-errors: 2 passed.
• Complete upstream suite: 2,578 passed, 12 skipped, with the four existing
  deprecation warnings in tests/test_entrypoint.py.
• git diff --check: passed.
• The project pre-commit executable was not available in the isolated local
  environment; GitHub CI should independently run the configured hooks.

Review note

This change was developed and validated in a concentrated session rather than
exercised over a long period in production. It has focused public-API regression
coverage, but careful human review of the implementation and assumptions is
requested.

AI assistance

This pull request was prepared with ChatGPT Codex using GPT-5.6 Sol with high
reasoning effort. Codex assisted with analysis, implementation, branch
isolation, and test execution. 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.