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/telegram.py

@@ -157,7 +157,8 @@ class TelegramMessageConverter(abstract_platform_adapter.AbstractMessageConverte
 
             message_components.append(
                 platform_message.Image(
-                    base64=f'data:{file_format};base64,{base64.b64encode(file_bytes).decode("utf-8")}'
+                    url=file.file_path,
+                    base64=f'data:{file_format};base64,{base64.b64encode(file_bytes).decode("utf-8")}',
                 )
             )