Improve TranslationProvider implementation with modern Java syntax and proper stub

[coderabbitai]

Description

The current TranslationProvider implementation in src/test/java/com/flowingcode/vaadin/addons/cronexpressioneditor/TranslationProvider.java can be improved with modern Java syntax and a proper stub implementation.

Current Issues

1. Uses legacy Collections.unmodifiableList(Arrays.asList()) instead of modern List.of()
2. Returns null from getTranslation() method which may cause issues
3. Contains TODO comment indicating incomplete implementation

Suggested Improvements

As suggested by @javier-godoy in PR #1:

@Override
public List<Locale> getProvidedLocales() {
  return List.of(Locale.ENGLISH);
}

@Override
public String getTranslation(String key, Locale locale, Object... params) {
  // Stub implementation for demo purposes
  return "\!"+key;
}

References

• PR: Initial implementation #1
• Comment: Initial implementation #1 (comment)

Acceptance Criteria

• Replace Collections.unmodifiableList(Arrays.asList(Locale.ENGLISH)) with List.of(Locale.ENGLISH)
• Implement proper stub for getTranslation() method that returns "\!" + key
• Remove TODO comment
• Ensure all tests still pass