🧪 [testing improvement] Add comprehensive tests for append_line_once

[Ch3fUlrich]

🎯 What: Added tests for the previously untested append_line_once bash function in lib/linux/install.sh.
📊 Coverage: The tests now cover:

• Idempotency (preventing duplicate lines across multiple calls).
• Proper formatting of appended text (joining arguments and adding the # added by AutoOS header).
• Automatic creation of parent directories when the target file path does not exist.
  ✨ Result: Improved test coverage, ensuring that idempotent file appending behaves correctly under all typical operating conditions.

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PR created automatically by Jules for task 14165974929492723991 started by @Ch3fUlrich

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