fix: ImageURLPart typo (#3665)

* 修复新版本更新对不上格式的问题

entities.py生成的格式：{"type": "image_url", "image_url": {"url": "data:image/jpeg;base64,..."}}
ImageURLPart期望的格式：{"type": "image_url", "image_url": "data:image/jpeg;base64,..."}

* Revert "修复新版本更新对不上格式的问题"

This reverts commit 28b4791391.

* fix(core.agent): 修复ImageURLPart的声明，修复pydantic校验失败的问题。

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Co-authored-by: piexian <piexian@users.noreply.github.com>
Co-authored-by: Dt8333 <lb0016@foxmail.com>

astrbot/core/agent/message.py

@@ -76,7 +76,7 @@ class ImageURLPart(ContentPart):
         """The ID of the image, to allow LLMs to distinguish different images."""
 
     type: str = "image_url"
-    image_url: str
+    image_url: ImageURL
 
 
 class AudioURLPart(ContentPart):