Exercise enclosure-like values without skipped tests

[claell]

Summary

• Replace enclosure-like value skips with explicit assertions.
• Verify removal values and removed_enclosing metadata across the full matrix.
• Assert exact output when addition is configured with no-enclosing.

Closes #579.

Validation

• pytest -p no:cacheprovider -W error tests/middleware_tests/test_enclosing.py: 926 passed, with no skips.
• Complete upstream test suite: 2,586 passed, 2 skipped, with the four existing deprecation warnings in tests/test_entrypoint.py.
• Complete pre-commit hook suite: passed.

Review note

This is a test-only change. The key judgment is that values beginning and ending with enclosure characters remain valid cases to test: removal interprets those boundary characters as enclosure, while addition configured with no-enclosing adds nothing and should be checked by exact equality.

AI assistance

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