refactor: simplify find_executable using shutil.which

[Dor-bl]

🎯 What: Replaced the manual executable lookup logic in find_executable within appium/webdriver/appium_service.py with Python's standard library shutil.which. Added unit tests in test/unit/webdriver/appium_service_test.py to verify behavior when an executable is found and when it is not.

💡 Why: shutil.which simplifies the implementation, improves readability, and leverages built-in, cross-platform resolution for executable files.

✅ Verification:

• Ran unit tests: PYTHONPATH=. uv run pytest test/unit/webdriver/appium_service_test.py (5 passed)
• Ran linter, type-checker, and formatting checks: make check (all passed)

✨ Result: Improved maintainability and cleanliness of find_executable without changing external behavior.

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PR created automatically by Jules for task 14895471279632432151 started by @Dor-bl

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