Python: small fix for CH serialization (#5738)

### Motivation and Context

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Noticed a small issue when ChatHistory is created with different types
of messages, and then serialized.
This fixed that and adds a test for that case.

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

python/semantic_kernel/connectors/ai/open_ai/contents/open_ai_chat_message_content.py

@@ -41,7 +41,10 @@ class OpenAIChatMessageContent(ChatMessageContent):
         if isinstance(tool_calls, list):
             for index, call in enumerate(tool_calls):
                 if not isinstance(call, ToolCall):
-                    tool_calls[index] = ToolCall.model_validate_json(call)
+                    if isinstance(call, dict):
+                        tool_calls[index] = ToolCall.model_validate(call)
+                    else:
+                        tool_calls[index] = ToolCall.model_validate_json(call)
             return tool_calls
         if isinstance(tool_calls, str):
             return [ToolCall.model_validate_json(call) for call in tool_calls.split("|")]
@@ -53,6 +56,8 @@ class OpenAIChatMessageContent(ChatMessageContent):
             return None
         if isinstance(function_call, FunctionCall):
             return function_call
+        if isinstance(function_call, dict):
+            return FunctionCall.model_validate(function_call)
         return FunctionCall.model_validate_json(function_call)
 
     @staticmethod

python/semantic_kernel/contents/chat_history.py

@@ -6,6 +6,7 @@ from typing import Any, Iterator, List
 from xml.etree.ElementTree import Element, tostring
 
 from defusedxml.ElementTree import XML, ParseError
+from pydantic import field_validator
 
 from semantic_kernel.contents.chat_message_content import ChatMessageContent
 from semantic_kernel.contents.chat_message_content_base import ChatMessageContentBase
@@ -34,7 +35,7 @@ class ChatHistory(KernelBaseModel):
         messages (List[ChatMessageContent]): The list of chat messages in the history.
     """
 
-    messages: list["ChatMessageContent"]
+    messages: list[ChatMessageContent]
     message_type: TYPES_CHAT_MESSAGE_CONTENT = CHAT_MESSAGE_CONTENT
 
     def __init__(self, **data: Any):
@@ -75,6 +76,19 @@ class ChatHistory(KernelBaseModel):
             data["messages"] = []
         super().__init__(**data)
 
+    @field_validator("messages", mode="before")
+    @classmethod
+    def _validate_messages(cls, messages: List[ChatMessageContent]) -> List[ChatMessageContent]:
+        if not messages:
+            return messages
+        out_msgs: List[ChatMessageContent] = []
+        for message in messages:
+            if isinstance(message, dict):
+                out_msgs.append(ChatMessageContentBase.from_dict(message))
+            else:
+                out_msgs.append(message)
+        return out_msgs
+
     def add_system_message(self, content: str, **kwargs: Any) -> None:
         """Add a system message to the chat history."""
         self.add_message(message=self._prepare_for_add(ChatRole.SYSTEM, content, **kwargs))

python/tests/unit/contents/test_chat_history.py

@@ -49,7 +49,7 @@ def test_add_system_message(chat_history: ChatHistory):
     assert chat_history.messages[-1].role == ChatRole.SYSTEM
 
 
-def test_add_system_message_at_init(chat_history: ChatHistory):
+def test_add_system_message_at_init():
     content = "System message"
     chat_history = ChatHistory(system_message=content)
     assert chat_history.messages[-1].content == content
@@ -190,6 +190,17 @@ def test_serialize():  # ignore: E501
     )
 
 
+def test_serialize_and_deserialize_to_chat_history_mixed_content():
+    system_msg = "a test system prompt"
+    msgs = [ChatMessageContent(role=ChatRole.USER, content=f"Message {i}") for i in range(3)]
+    msgs.extend([OpenAIChatMessageContent(role=ChatRole.USER, content=f"Message {i}") for i in range(3)])
+    msgs.extend([AzureChatMessageContent(role=ChatRole.USER, content=f"Message {i}") for i in range(3)])
+    chat_history = ChatHistory(messages=msgs, system_message=system_msg)
+    json_str = chat_history.serialize()
+    new_chat_history = ChatHistory.restore_chat_history(json_str)
+    assert new_chat_history == chat_history
+
+
 def test_serialize_and_deserialize_to_chat_history():
     system_msg = "a test system prompt"
     msgs = [ChatMessageContent(role=ChatRole.USER, content=f"Message {i}") for i in range(3)]