Summary of Changes

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This pull request addresses two AI-identified code quality findings by implementing targeted improvements in string handling and test coverage. The core change involves refining the FromCp437 trait's behavior to more efficiently manage memory through strategic use of std::borrow::Cow, thereby reducing redundant allocations. Additionally, a test case has been expanded to ensure robust validation across the entire range of u8 values.

Highlights

• Performance Optimization: The FromCp437 trait implementation in src/cp437.rs has been optimized to reduce unnecessary memory allocations. For ASCII strings, it now directly returns a std::borrow::Cow::Borrowed reference instead of creating an owned string. For non-ASCII strings, it streamlines the return to std::borrow::Cow::Owned.
• Test Coverage Improvement: The to_char_valid test in src/cp437.rs has been updated to iterate through the full range of u8 values (0 to 255, inclusive) by changing u8::MIN..u8::MAX to u8::MIN..=u8::MAX, ensuring comprehensive testing of the to_char function.