🧪 Add tests for Shell::from_path

[bashandbone]

🎯 What: The testing gap addressed
The Shell::from_path (and Shell::parse_shell_from_path) function in src/shells.rs was missing unit tests, leaving its behavior mapping executable paths to enum variants untested.

📊 Coverage: What scenarios are now tested

• Supported shells (bash, zsh, fish, powershell/pwsh, elvish, nu/nushell) with and without path components and extensions (.exe).
• Unrecognized shells (sh, cmd.exe, python, etc.) yielding None.
• Edge cases like empty strings, root paths (/), current dir (.), parent dir (..), and dotfiles (.bash) yielding None.

✨ Result: The improvement in test coverage
src/shells.rs now has a comprehensive test suite for its primary parsing function, ensuring regressions aren't introduced when modifying shell support or path parsing logic.

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PR created automatically by Jules for task 2450735061863755547 started by @bashandbone

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[Copilot]

Pull request overview

Adds a focused unit test suite for Shell::from_path in src/shells.rs, improving confidence that executable-path parsing continues to map to the correct Shell variants as shell support evolves.

Changes:

• Added unit tests covering recognized shell executable names (with/without directories and .exe extensions).
• Added unit tests ensuring unknown/invalid/edge-case paths return None.

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