Add unit tests for unicode whitespace and zero-width spaces in sentinel normalization

[JohnnyWilson16]

Context

As identified in the Contributor Roadmap (Level 1: Quick Wins), web-scraped and user-entered text often contains invisible unicode whitespace that can interfere with sentinel detection if not properly tested.

Task

Add unit tests in tests/test_strings.py covering unicode whitespace characters (such as non-breaking space \u00A0 and zero-width space \u200B) to verify they are handled as expected during sentinel normalization and whitespace stripping.

Acceptance Criteria

• Add test case(s) in tests/test_strings.py testing strings with \u00A0 (non-breaking space) and \u200B (zero-width space).
• Test interactions with standard sentinels (e.g., "\u00A0N/A\u00A0" and "\u200BNA\u200B").
• Ensure tests verify both strip_whitespace=True and default fd.clean() behavior.
• Tests execute fast and pass cleanly with pytest tests/test_strings.py --no-cov.
• Code adheres to style guidelines (ruff check tests/test_strings.py).

Pointers

• Test file: tests/test_strings.py
• Sentinel step implementation: src/freshdata/steps/sentinels.py

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