FIX Coerce enum parameters for direct target creation

[romanlutz]

Description

Target creation through the backend can bypass registry coercion for classes that accept base configuration through **kwargs. As a result, enum-valued API parameters can remain raw strings instead of the values expected by target constructors.

This change extends the shared Parameter contract to coerce enum and optional scalar values, then applies that contract before direct target construction. Undeclared **kwargs and registry reference resolution remain unchanged, and no target-specific implementations are modified.

Tests and Documentation

• Added unit coverage for enum and optional-enum coercion in parameter metadata, registry resolution, target registration, and backend target creation.
• Focused unit tests: 192 passed.
• Changed-file pre-commit hooks passed, including Ruff and ty type checking.
• Documentation: N/A; this fixes internal coercion behavior without changing the public API shape.
• JupyText: N/A.