Chore: PromptMessage is not an abstract base class (#15965)

api/core/model_runtime/docs/en_US/interfaces.md

 The base class for all Role message bodies, used only for parameter declaration and cannot be initialized.
 
 ```python
-class PromptMessage(ABC, BaseModel):
+class PromptMessage(BaseModel):
     """
     Model class for prompt message.
     """

api/core/model_runtime/docs/zh_Hans/interfaces.md

 所有 Role 消息体的基类，仅作为参数声明用，不可初始化。
 
 ```python
-class PromptMessage(ABC, BaseModel):
+class PromptMessage(BaseModel):
     """
     Model class for prompt message.
     """

api/core/model_runtime/entities/__init__.py

     "ModelPropertyKey",
     "MultiModalPromptMessageContent",
     "PromptMessage",
-    "PromptMessage",
     "PromptMessageContent",
     "PromptMessageContentType",
     "PromptMessageRole",
-    "PromptMessageRole",
     "PromptMessageTool",
     "SystemPromptMessage",
     "TextPromptMessageContent",

api/core/model_runtime/entities/message_entities.py

-from abc import ABC
 from collections.abc import Sequence
 from enum import Enum, StrEnum
 from typing import Optional
     type: PromptMessageContentType = PromptMessageContentType.DOCUMENT
 
 
-class PromptMessage(ABC, BaseModel):
+class PromptMessage(BaseModel):
     """
     Model class for prompt message.
     """