Refactor SpecModel to new class hierarchy

[dwikler]

Title:
Refactor SpecModel to new class hierarchy (BaseSpecModel, TableSpecModel, SectionSpecModel)

Description:

• Create a new base class for spec models with flexible metadata and content types.
• Refactor current SpecModel to TableSpecModel (inherits from BaseSpecModel).
• Create SectionSpecModel for explanatory sections.
• Update all factories, builders, and registries to use the new model classes.
• Update all usages and tests accordingly.