docs: add missing docs

ref: N25B-115

src/control_backend/schemas/chat_history.py

@@ -2,9 +2,22 @@ from pydantic import BaseModel
 
 
 class ChatMessage(BaseModel):
+    """
+    Represents a single message in a conversation.
+
+    :ivar role: The role of the speaker (e.g., 'user', 'assistant').
+    :ivar content: The text content of the message.
+    """
+
     role: str
     content: str
 
 
 class ChatHistory(BaseModel):
+    """
+    Represents a sequence of chat messages, forming a conversation history.
+
+    :ivar messages: An ordered list of :class:`ChatMessage` objects.
+    """
+
     messages: list[ChatMessage]