Harden RC7 API response contracts

backend/app/schemas/assistant.py

@@ -30,6 +30,12 @@ class AssistantModelRead(BaseModel):
     capabilities: list[str] = Field(default_factory=list)
 
 
+class AssistantModelList(BaseModel):
+    items: list[AssistantModelRead]
+    total: int
+    default_model: str | None = None
+
+
 class AssistantStatus(BaseModel):
     enabled: bool
     reachable: bool