feat(platform): pass original image URL to Image component in 6 adapters (#2362)

Preserve the platform CDN URL in Image.url alongside base64 data,
enabling plugins to use ContentElement.from_image_url() for direct
vision API access without redundant local download.

- aiocqhttp: use msg_data["data"]["url"] and msg.data["url"]
- discord: use attachment.url
- telegram: use file.file_path
- slack: use pic_url
- wecom: use picurl
- qqofficial: use pic_url

Satori adapter already follows this pattern (satori.py:168).
The change is purely additive — base64 is preserved for backward
compatibility, and get_bytes() priority (url → base64 → path)
ensures plugins can choose the optimal path.

Closes #2355

Co-authored-by: douxt <8429023+douxt@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>

src/langbot/pkg/platform/sources/slack.py

@@ -47,7 +47,7 @@ class SlackMessageConverter(abstract_platform_adapter.AbstractMessageConverter):
         yiri_msg_list.append(platform_message.Source(id=message_id, time=datetime.datetime.now()))
         if pic_url is not None:
             base64_url = await image.get_slack_image_to_base64(pic_url=pic_url, bot_token=bot.bot_token)
-            yiri_msg_list.append(platform_message.Image(base64=base64_url))
+            yiri_msg_list.append(platform_message.Image(url=pic_url, base64=base64_url))
 
         yiri_msg_list.append(platform_message.Plain(text=message))
         chain = platform_message.MessageChain(yiri_msg_list)