Test duplicate key namespaces

[claell]

Summary

• Cover strict duplicate detection for @string blocks.
• Verify that entries and strings use separate key namespaces.
• Keep the change test-only and document the behavior each regression protects.

Closes #575.

Validation

• pytest -p no:cacheprovider -W error tests/test_library.py: 10 passed.
• Complete upstream test suite: 2,578 passed, 12 skipped, with the four existing deprecation warnings in tests/test_entrypoint.py.
• Complete pre-commit hook suite: passed.

Review note

This PR makes currently passing behavior explicit; it does not change production code. Maintainer review should confirm that separate entry and string namespaces are the intended public contract.

AI assistance

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