chore: add more test to Survey Manager

[mattinannt]

This pull request introduces several changes to enhance the testability and accessibility of methods in the SurveyManager and UserManager classes. The most important changes include making key methods in SurveyManager publicly accessible for testing purposes, adding detailed documentation for these methods, and introducing a test-only getter in UserManager.

Changes to SurveyManager:

• Made the filterSurveysBasedOnDisplayType method public and annotated it with @JvmStatic to allow external access and testing. Added detailed documentation for the method's parameters and return value.
• Made the filterSurveysBasedOnRecontactDays method public and annotated it with @JvmStatic. Added documentation to describe the method's purpose and behavior.
• Introduced a new public invokeShouldDisplayBasedOnPercentage method annotated with @JvmStatic as a test helper for invoking the private shouldDisplayBasedOnPercentage method.
• Made the getLanguageCode method public and annotated it with @JvmStatic. Added comprehensive documentation to explain its behavior and edge cases.

Changes to UserManager:

• Changed the setter for lastDisplayedAt from private to internal to allow controlled access within the module.
• Added a new internal getLastDisplayedAtForTesting method to provide test-only access to the lastDisplayedAt property.

[sonarqubecloud]

Quality Gate failed

Failed conditions
50.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud