Address PR feedback: Enhance type safety and add tests for multimodal validation

Co-Authored-By: Joe Moura <joao@crewai.com>

src/crewai/utilities/events/llm_events.py

@@ -1,5 +1,5 @@
 from enum import Enum
-from typing import Any, Dict, List, Optional, Union
+from typing import Any, Dict, List, Literal, Optional, Union
 
 from crewai.utilities.events.base_events import CrewEvent
 
@@ -11,11 +11,47 @@ class LLMCallType(Enum):
     LLM_CALL = "llm_call"
 
 
+class ContentType(str, Enum):
+    """Types of content in multimodal messages"""
+    
+    TEXT = "text"
+    IMAGE_URL = "image_url"
+
+
 class LLMCallStartedEvent(CrewEvent):
     """Event emitted when a LLM call starts"""
 
     type: str = "llm_call_started"
-    messages: Union[str, List[Dict[str, Any]]]
+    messages: Union[
+        str,
+        List[Union[
+            str,
+            Dict[str, Union[
+                str,
+                List[Dict[str, Union[
+                    str,
+                    Dict[Literal["url"], str]
+                ]]]
+            ]]
+        ]]
+    ]
+    """
+    Supports both string messages and structured messages including multimodal content.
+    Formats supported:
+    1. Simple string: "This is a message"
+    2. List of message objects: [{"role": "user", "content": "Hello"}]
+    3. Mixed list with strings and objects: ["Simple message", {"role": "user", "content": "Hello"}]
+    4. Multimodal format:
+    {
+        'role': str,
+        'content': List[
+            Union[
+                Dict[Literal["type", "text"], str],
+                Dict[Literal["type", "image_url"], Dict[str, str]]
+            ]
+        ]
+    }
+    """
     tools: Optional[List[dict]] = None
     callbacks: Optional[List[Any]] = None
     available_functions: Optional[Dict[str, Any]] = None