FIX Coerce enum parameters for direct target creation

[romanlutz]

Description

Backend target creation could bypass registry coercion for classes that forward base configuration through **kwargs, leaving enum-valued API parameters as raw strings. The service also duplicated registry metadata lookup and coercion logic.

This change keeps enum and optional-scalar coercion in the shared Parameter contract, makes constructor derivation follow explicitly forwarded parent constructors in MRO order, and routes all backend target construction through TargetRegistry.create_instance. OpenAI target constructors declare their forwarding behavior, so base parameters such as endpoint, model_name, and api_key become first-class registry metadata while strict unknown-parameter rejection remains intact. Open-ended keyword bags that do not forward to a parent are not treated as inherited constructor parameters.

Tests and Documentation

• Added coverage for enum and optional-enum coercion, forwarded constructor metadata/coercion, strict unknown-parameter rejection, target registration, and backend registry delegation.
• Registry, target-service, and constructor-contract suite: 424 passed.
• Parameter model suite: 88 passed.
• Changed-file pre-commit hooks passed, including Ruff and ty type checking.
• Documentation: N/A; this fixes internal construction and coercion behavior without changing the public API shape.
• JupyText: N/A.