refactor(optim-config): split parsing (reading) from cross-validation

[Juliette-Gerbaux]

Process ID

Process: GP-02

Description

Splits gems_craft/optim_config/parsing.py into a reading module and a validation module:

• parsing.py keeps the Pydantic schema (OptimConfig, ScenarioScopeConfig, ResolutionConfig, etc.) and load_optim_config().
• New validation.py holds validate_optim_config() and its _check_* helpers, which cross-validate a parsed OptimConfig against a resolved System.

All call sites that imported validate_optim_config directly from .parsing (gems_runner/study/runner.py and three test files) were updated to import it from .validation instead; the package's __init__.py re-exports are unchanged.

Also adds TODO markers where the same reading/validation mixing recurs elsewhere in the codebase, for a future follow-up pass:

• study/resolve_components.py: consistency_check() and the inline missing-parameter/property checks in _resolve_component()
• model/resolve_library.py: the _forbid_* expression-validation helpers embedded in the resolution pass
• study/study.py: Study.check_consistency() duplicates/overlaps with resolve_components.consistency_check() without being consolidated

Impact Analysis

• Affected modules: optim_config/ (split), plus study/ and model/ (TODO comments only, no behavior change).
• No solver output values change — this is a pure code-organization refactor; all moved functions are byte-for-byte identical, only import paths changed.

Checklist

• Unit tests pass (pytest)
• Type checking passes (mypy)
• Formatting passes (black, isort)
• pyproject.toml version bumped if applicable
• AGENTS.md reviewed for impact and updated if needed