test(strings): cover Unicode whitespace and sentinel boundaries

[WilliamK112]

Add six requested Unicode regression cases to tests/test_strings.py for object and pandas string columns. They cover default fd.clean() and explicit stripping, NBSP-padded sentinels/values, zero-width-space content, internal spacing, repair counts, and stripping disabled. Populated row identifiers keep sentinel-to-missing conversions observable without dropping rows.

NBSP is stripped at cell boundaries and can expose a sentinel; U+200B follows the existing Python whitespace rules and remains content. This PR changes tests only.

Fixes #498.

Validation:

• pytest tests/test_strings.py --no-cov: all 29 cases pass on pandas 2.3.3; 26 pass and three existing Arrow cases skip on pandas 1.5.3.
• Full fast lane: 7,608 passed, 49 skipped, five failures reproduced on clean main@ba3f01f in this Windows environment, 94.98% coverage.
• Full Ruff check, Linux-platform mypy with CI's NumPy cap, and diff checks pass.

Draft pending #518: the floating Polars 2.0 matrix has an existing all-columns-dropped parity failure, addressed by that PR. Keep this test-only scope separate; refresh from main and mark ready after the fix lands and hosted checks pass.

[coderabbitai]

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[JohnnyWilson16]

LGTM! Thorough test coverage for #498. Correctly verifies U+00A0 stripping, U+200B preservation, and audit report counts across string and object dtypes without test flakiness. Ready to merge.