Refactor Engine config access to unified pattern

[kettasoft]

This PR refactors the abstract Engine class to unify how configuration values are accessed across all engine subclasses.

• Updated methods like getOperatorsFromConfig, isIgnoredEmptyValuesFromConfig, getAllowedFieldsFromConfig, isStrictFromConfig to use a consistent pattern:

  return config("filterable.engines.{$this->getEngineName()}.<key>", <default>);

• Removed unused imports from the Engine abstract class.

• Simplifies maintenance and avoids repeated boilerplate for fetching engine-specific configuration.

This refactor does not change runtime behavior, only standardizes the configuration access pattern.