[Junie]: Fix WARN for incorrect completeCiteCommand

[github-actions]

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• 🔗 Issue: Fixes: WARN if wrong completeCiteCommand #723

📊 Junie Summary:

Implemented a warning for invalid cite commands in the external preferences tab.

The following changes were made:

• Log level changed to WARN: In CitationCommandString.from, the log level was upgraded from INFO to WARN when an invalid completeCiteCommand is encountered.
• Validation in UI: Added a FunctionBasedValidator to ExternalTabViewModel that checks if the cite command contains both key1 and key2 in the correct order.
• New Localization String: Added The cite command should contain '%0' and '%1'. to JabRef_en.properties.
• UI Integration: Connected the validator to the citeCommand text field in ExternalTab using validationVisualizer.
• Accessibility: Made CITE_KEY1 and CITE_KEY2 constants public in CitationCommandString for use in validation logic.
• Tests Updated: Added test cases to CitationCommandStringTest to cover invalid cite command scenarios and verified they pass.
• Code Quality: Ensured checkstyle compliance by adjusting the declaration order of constants in CitationCommandString.