Refactor filename sanitization logic and expand unit tests

[herewithme]

• Updated the bea_sanitize_file_name function to improve extension handling, ensuring only the suffix is removed.
• Enhanced unit tests to cover various filename scenarios, including accented characters, underscores to dashes conversion, and upload-style filenames.
• Added data providers for better test organization and clarity.

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Note

Medium Risk
Changes the filename sanitization output for multi-dot filenames by stripping the extension only when it appears as the final suffix, which could affect stored upload filenames/URLs. Risk is mitigated by significantly expanded unit test coverage across common and edge-case filenames.

Overview
Filename sanitization logic is tightened so the plugin derives the extension via pathinfo() and removes it only as a true end-of-string suffix (avoiding accidental removal of matching substrings in multi-dot names), then reattaches it unchanged.

Unit tests are massively expanded and reorganized using data providers to cover accents/transliteration interactions, underscore/space handling, special character stripping (including @, €, ©), UTF-8/non‑Latin names, and multi-dot “fake extension” cases from WordPress core.

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