.Net: Chat history serialization for token counting is broken in StepwisePlanner

[neodenit]

Describe the bug
When tokens are counted in StepwisePlanner, messages serialize incorrectly: Code URL

The outcome looks like this:

Microsoft.SemanticKernel.AI.ChatCompletion.ChatHistory+ChatMessage
Microsoft.SemanticKernel.AI.ChatCompletion.ChatHistory+ChatMessage
Microsoft.SemanticKernel.AI.ChatCompletion.ChatHistory+ChatMessage
Microsoft.SemanticKernel.AI.ChatCompletion.ChatHistory+ChatMessage

To Reproduce
Steps to reproduce the behavior:

1. Put a breakpoint in GetChatHistoryTokens
2. Run StepwisePlanner
3. See messages string content

Expected behavior
Message content should be concatenated.

Screenshots
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Platform

• OS: Windows
• IDE: Visual Studio
• Language: C#
• Source: NuGet package version 0.23.230906.2-preview

Additional context
Add any other context about the problem here.