feat: implement scenario comparison validation and API endpoint with comprehensive unit tests

services/exceptions.py

@@ -1,5 +1,8 @@
 """Domain-level exceptions for service and repository layers."""
 
+from dataclasses import dataclass
+from typing import Sequence
+
 
 class EntityNotFoundError(Exception):
     """Raised when a requested entity cannot be located."""
@@ -7,3 +10,15 @@ class EntityNotFoundError(Exception):
 
 class EntityConflictError(Exception):
     """Raised when attempting to create or update an entity that violates uniqueness."""
+
+
+@dataclass(eq=False)
+class ScenarioValidationError(Exception):
+    """Raised when scenarios fail comparison validation rules."""
+
+    code: str
+    message: str
+    scenario_ids: Sequence[int] | None = None
+
+    def __str__(self) -> str:  # pragma: no cover - mirrors message for logging
+        return self.message