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

schemas/scenario.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 from datetime import date, datetime
 
-from pydantic import BaseModel, ConfigDict, field_validator
+from pydantic import BaseModel, ConfigDict, field_validator, model_validator
 
 from models import ResourceType, ScenarioStatus
 
@@ -64,3 +64,24 @@ class ScenarioRead(ScenarioBase):
     updated_at: datetime
 
     model_config = ConfigDict(from_attributes=True)
+
+
+class ScenarioComparisonRequest(BaseModel):
+    scenario_ids: list[int]
+
+    model_config = ConfigDict(extra="forbid")
+
+    @model_validator(mode="after")
+    def ensure_minimum_ids(self) -> "ScenarioComparisonRequest":
+        unique_ids: list[int] = list(dict.fromkeys(self.scenario_ids))
+        if len(unique_ids) < 2:
+            raise ValueError("At least two unique scenario identifiers are required for comparison.")
+        self.scenario_ids = unique_ids
+        return self
+
+
+class ScenarioComparisonResponse(BaseModel):
+    project_id: int
+    scenarios: list[ScenarioRead]
+
+    model_config = ConfigDict(from_attributes=True)