Improve escape handling and add comprehensive test coverage for edge …

…cases

- Fix surrogate pair handling to preserve emoji as single characters instead of escaping
- Add uppercase hex formatting for unpaired surrogates in escape function
- Validate hex digits in unicode escape sequences during unescape
- Handle unpaired surrogates by returning replacement characters
- Add literal 'u' handling when \u lacks sufficient characters
- Add test coverage for array operations in diff/patch (replace, remove