test: wrote additional test

[AsfhtgkDavid]

This pull request significantly increases test coverage across several modules by adding comprehensive unit tests for core functionality related to shell handling, command parsing, alias expansion, and string similarity. The new tests ensure correct behavior for various edge cases and input scenarios, improving code reliability and maintainability.

Expanded Unit Test Coverage

• Added tests for the NativeRule enum, including parsing from strings and the fix_native logic for different command scenarios.
• Added tests for the CommandOutput and Command structs, covering construction, parsing, and handling of quoted and empty commands.
• Added tests for string similarity and distance functions (damerau_levenshtein_distance, string_similarity) and alias expansion, ensuring correct results for identical, similar, and completely different strings, as well as empty inputs.

Shell-related Functionality Testing

• Added tests for the Shell enum parsing from strings and for the generation of shell-specific function definitions across Bash, Zsh, and Fish.
• Added tests for shell-specific alias parsing and shell function generation in bash.rs, zsh.rs, and fish.rs, including handling of environment variables and different quoting styles. [1] [2] [3]

Shell Detection Logic

• Added tests for shell detection by environment variable in helpers.rs, covering valid, invalid, and unset cases for SH_SHELL.

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