text: Title: Add optional timestamp to SystemPromptPart
  Body: UserPromptPart, ToolReturnPart, RetryPromptPart all include a timestamp attribute.
  As do TextPart, ToolCallPart.
  
  SystemPromptPart is the only part type in the ModelRequestPart that does not have a timestamp. 
  
   There are times that we create a system prompt instance from a template, and this instantiation is done at particular point in time. There are also times when this happens in other systems, and that history is passed to our pydantic-ai service, and the timestamp is consider a required attribute as part of retaining an immutable timestamped sequence of entries related to a user/ai chat.
  
  Can we please add 
  ```
      timestamp: datetime = field(default_factory=_now_utc)
  ```
  to SystemPromptPart to bring it in line with the other parts