feat(unique fields) #32172 : Turn on unique field validation via DB by default

[jcastro-dotcms]

Proposed Changes

This pull request includes updates to enable database-level unique field validation by default and improve the clarity and functionality of the UniqueFieldsValidationInitializer class. The changes include enabling a feature flag, enhancing logging, and refining code readability and documentation.

Feature flag and default behavior updates:

• Updated the FEATURE_FLAG_DB_UNIQUE_FIELD_VALIDATION in ESContentletAPIImpl to enable database-level unique field validation by default. (dotCMS/src/main/java/com/dotcms/content/elasticsearch/business/ESContentletAPIImpl.java, dotCMS/src/main/java/com/dotcms/content/elasticsearch/business/ESContentletAPIImpl.javaL244-R244)

Improvements to UniqueFieldsValidationInitializer:

• Enhanced the class-level Javadoc to provide a clearer explanation of its responsibilities and behavior during application startup. (dotCMS/src/main/java/com/dotcms/contenttype/business/uniquefields/UniqueFieldsValidationInitializer.java, dotCMS/src/main/java/com/dotcms/contenttype/business/uniquefields/UniqueFieldsValidationInitializer.javaL16-R28)
• Added logging statements to indicate when the unique fields table is being created, populated, or dropped, improving traceability. (dotCMS/src/main/java/com/dotcms/contenttype/business/uniquefields/UniqueFieldsValidationInitializer.java, dotCMS/src/main/java/com/dotcms/contenttype/business/uniquefields/UniqueFieldsValidationInitializer.javaL40-R55)
• Refactored variable declarations for better readability (e.g., adding final to variables) and improved exception handling. (dotCMS/src/main/java/com/dotcms/contenttype/business/uniquefields/UniqueFieldsValidationInitializer.java, dotCMS/src/main/java/com/dotcms/contenttype/business/uniquefields/UniqueFieldsValidationInitializer.javaL40-R55)
• Removed unnecessary blank lines for cleaner code formatting. (dotCMS/src/main/java/com/dotcms/contenttype/business/uniquefields/UniqueFieldsValidationInitializer.java, dotCMS/src/main/java/com/dotcms/contenttype/business/uniquefields/UniqueFieldsValidationInitializer.javaR67)