fix: send messages in chat format in MCP add_memory tool (#1275)

The /product/add endpoint expects messages as a list of chat message
objects, not a plain string. When passed a string, the SimpleStruct
MemReader silently skips it and no memory is stored.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: CaralHsi <caralhsi@gmail.com>

examples/mem_mcp/simple_fastmcp_serve.py

@@ -23,7 +23,7 @@ def add_memory(memory_content: str, user_id: str, cube_id: str | None = None):
     """Add memory using the Server API."""
     payload = {
         "user_id": user_id,
-        "messages": memory_content,
+        "messages": [{"role": "user", "content": memory_content}],
         "writable_cube_ids": [cube_id] if cube_id else None,
     }
     try: