test: add unit tests for get_npm

[Dor-bl]

🎯 What: Address missing unit tests for get_npm in appium/webdriver/appium_service.py.

📊 Coverage:

• get_npm() success path on non-Windows (POSIX) operating systems.
• get_npm() success path on Windows operating systems (sys.platform == 'win32').
• get_npm() raising AppiumServiceError when the npm executable is not found in PATH.

✨ Result: Improved unit test coverage for appium_service.py functions and ensured get_npm behavior is thoroughly tested.

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PR created automatically by Jules for task 16867738139041648940 started by @Dor-bl

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