fix(wecom_bot): revert callback image cap to 512KB (2MB inline body rejected by WeCom)

The 2MB cap (matching the long-connection upload path) does not work for
the callback path: there the whole image is base64-embedded in an
AES-encrypted body returned on every poll. A ~1.5MB image (base64 ~2.1MB,
encrypted ~2.8MB) makes WeCom reject the finish packet and poll forever,
which also surfaces as a truncated text bubble and WeCom's own timeout
error. Cap well below that at 512KB so the finish packet is accepted.

Co-authored-by: Cursor <cursoragent@cursor.com>

channel/wecom_bot/wecom_bot_channel.py

@@ -892,9 +892,12 @@ class WecomBotChannel(ChatChannel):
         if not local_path:
             return None
 
-        # Keep consistent with the long-connection upload path (2MB limit):
-        # only compress when the image exceeds the cap, otherwise send as-is.
-        callback_max_size = 2 * 1024 * 1024
+        # Unlike the long-connection path (which uploads and sends only a tiny
+        # media_id), the callback reply embeds the whole image as base64 inside
+        # an AES-encrypted body that is returned on EVERY poll. Empirically a
+        # ~1.5MB image (base64 ~2.1MB, encrypted ~2.8MB) makes WeCom reject the
+        # finish packet and poll forever, so cap well below that.
+        callback_max_size = 512 * 1024
         if os.path.getsize(local_path) > callback_max_size:
             compressed = self._compress_image(local_path, callback_max_size)
             if compressed: