fix(review): replace unreachable normalization with assertions in message merge

- Replace defensive normalization with explicit type assertions
- Assertions enforce invariants and surface violations clearly

Refs: #2021

Author: giulio-leone

src/smolagents/models.py

@@ -380,17 +380,15 @@ def get_clean_message_list(
             assert isinstance(message.content, list), "Error: wrong content:" + str(message.content)
             if flatten_messages_as_text:
                 # In flatten mode, output content is stored as a plain string.
-                # Normalize it if needed before concatenating.
-                if isinstance(output_message_list[-1]["content"], list):
-                    output_message_list[-1]["content"] = output_message_list[-1]["content"][0]["text"]
+                assert isinstance(
+                    output_message_list[-1]["content"], str
+                ), "Error: expected string content in flatten mode"
                 output_message_list[-1]["content"] += "\n" + message.content[0]["text"]
             else:
                 # In structured mode, output content is stored as a list of dicts.
-                # Normalize it if needed before merging.
-                if isinstance(output_message_list[-1]["content"], str):
-                    output_message_list[-1]["content"] = [
-                        {"type": "text", "text": output_message_list[-1]["content"]}
-                    ]
+                assert isinstance(
+                    output_message_list[-1]["content"], list
+                ), "Error: expected list content in structured mode"
                 for el in message.content:
                     if el["type"] == "text" and output_message_list[-1]["content"][-1]["type"] == "text":
                         # Merge consecutive text messages rather than creating new ones