Fix mem0 external memory format issue #3152

- Convert string values to proper message format for mem0 API
- mem0 API expects messages as list of objects with role/content fields
- Add comprehensive tests for external memory type and message formatting
- Add reproduction script to verify the fix works
- Resolves 'Expected a list of items but got type str' error

Co-Authored-By: Jo\u00E3o <joao@crewai.com>

reproduce_issue.py

@@ -0,0 +1,55 @@
+"""
+Reproduction script for issue #3152 - mem0 external memory format error
+Based on the code provided in the GitHub issue
+"""
+
+import os
+from crewai import Agent, Task, Crew
+from crewai.memory.external.external_memory import ExternalMemory
+
+def test_mem0_external_memory():
+    """Test that reproduces the mem0 external memory format error"""
+    
+    embedder_config = {
+        "provider": "mem0",
+        "config": {
+            "user_id": "test_user_123",
+        }
+    }
+    
+    external_memory = ExternalMemory(embedder_config=embedder_config)
+    
+    agent = Agent(
+        role="Test Agent",
+        goal="Test external memory functionality",
+        backstory="A test agent for reproducing the mem0 issue",
+        verbose=True
+    )
+    
+    task = Task(
+        description="Test task for external memory",
+        expected_output="Test output",
+        agent=agent
+    )
+    
+    crew = Crew(
+        agents=[agent],
+        tasks=[task],
+        external_memory=external_memory,
+        verbose=True
+    )
+    
+    print("Testing mem0 external memory integration...")
+    
+    try:
+        result = crew.kickoff()
+        print("SUCCESS: External memory integration worked!")
+        print(f"Result: {result}")
+    except Exception as e:
+        print(f"ERROR: {e}")
+        if "Expected a list of items but got type" in str(e):
+            print("CONFIRMED: This is the mem0 format error from issue #3152")
+        raise
+
+if __name__ == "__main__":
+    test_mem0_external_memory()