[Improvement]: Use typed class constants (public const <type>) instead of untyped const

[solverat]

Improvement description

To improve type safety and make the code more explicit, class constants should use typed constants where possible.

Instead of declaring constants without a type, we should explicitly define the type using public const .

Example

# before
const ATTRIBUTE_OPENDXP_STATIC_PAGE = '_opendxp_static_page';
# after
public const string ATTRIBUTE_OPENDXP_STATIC_PAGE = '_opendxp_static_page';