feat: add metric_weights and doc_weights to Scenario schema and dataclass

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

rag_eval/config/loader.py

@@ -62,6 +62,8 @@ def load_scenario(path: str | Path) -> Scenario:
         ),
         source_path=scenario_path,
         optimization_advisor=model.optimization_advisor,
+        metric_weights=dict(model.metric_weights),
+        doc_weights=dict(model.doc_weights),
     )
     # Run cross-field checks after all relative paths have been resolved.
     validate_scenario(scenario)