Fix validate_mc_scenarios error message; update load_optim_config docstring

validate_mc_scenarios was still converting indices to 1-based in its error
message after the 0-based switch. Error now reports the raw 0-based indices
and the correct range (0–N-1).

load_optim_config docstring replaced: removes the stale 'components_path'
reference and documents the two extra steps the function performs beyond
plain YAML parsing (relative path resolution and eager scenario_ids
population).

https://claude.ai/code/session_01EfjU9tBfdyZ9nZmQFCe5uU

Author: claude

src/gems/optim_config/parsing.py

@@ -286,10 +286,17 @@ class OptimConfig(ModifiedBaseModel):
 
 
 def load_optim_config(config_path: Path) -> Optional[OptimConfig]:
-    """Load optim-config.yml from the same directory as components_path.
+    """Load and fully resolve an ``optim-config.yml`` file.
 
-    Returns None if the file does not exist.
-    Raises ValueError on parsing or validation failure.
+    Returns ``None`` if the file does not exist.
+    Raises ``ValueError`` on any parsing, validation, or playlist I/O failure.
+
+    Beyond plain YAML parsing, this function:
+
+    - Resolves a relative ``playlist-file`` path against the directory that
+      contains the config file.
+    - Eagerly populates ``scenario_ids`` so that playlist file errors surface
+      immediately rather than at first use.
     """
     if not config_path.exists():
         return None

src/gems/study/scenario_builder.py

@@ -81,8 +81,8 @@ def validate_mc_scenarios(self, scenario_ids: List[int]) -> List[str]:
             out_of_bounds = [idx for idx in scenario_ids if idx >= len(arr)]
             if out_of_bounds:
                 errors.append(
-                    f"Scenario indices {[i + 1 for i in out_of_bounds]} (1-based) are not defined "
-                    f"for scenario group '{group}' (defined range: 1–{len(arr)})"
+                    f"Scenario indices {out_of_bounds} are not defined "
+                    f"for scenario group '{group}' (defined range: 0–{len(arr) - 1})"
                 )
         return errors