fix: replace struct with typed enum for proper validation dispatch

[PritamP20]

Declaration was a thin base struct with only _class, name, decorators, and location. Every time a typed declaration (ConceptDeclaration, AssetDeclaration, etc.) was added to a model, all its fields properties, superType, isAbstract got stripped away. Three methods in the codebase were working around this with hardcoded hacks.

Declaration is now a proper enum with 13 variants, dispatching on $class for deserialization and delegating to the inner struct for serialization.

Changes

• Replace Declaration struct with a typed enum (Concept, Asset, Participant, Transaction, Event, Enum, Map, and 6 scalar variants)
• Fix validate_no_circular_inheritance was hardcoded to only detect cycles between "Person" and "Employee", now works for any class names
• Fix as_concept_declaration and as_map_declaration were always returning None
• Remove hardcoded find_properties_for_declaration hack from ModelFile
• Simplify ModelFile::validate from ~30 lines of workarounds to a single line
• Add Clone to 15 structs required by the enum variants
• Enable 3 previously #[ignore]d tests that now pass correctly

Flags

• Breaking change to the Declaration type callers must now use Declaration::Concept(d) instead of the base struct literal
• All existing tests updated accordingly

Tested

25 tests pass, 0 failing

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary (README usage example predates this PR and uses incorrect API — tracked separately)
• Merging to main from fork:fix/declaration-typed-enum

[PritamP20]

@mttrbrts can you please review this once