fix(gateway): validate Slack image downloads before caching

Slack may return an HTML sign-in/redirect page instead of actual media
bytes (e.g. expired token, restricted file access). This adds two layers
of defense:

1. Content-Type check in slack.py rejects text/html responses early
2. Magic-byte validation in base.py's cache_image_from_bytes() rejects
   non-image data regardless of source platform

Also adds ValueError guards in wecom.py and email.py so the new
validation doesn't crash those adapters.

Closes #6829

gateway/platforms/email.py

@@ -195,7 +195,11 @@ def _extract_attachments(
 
         ext = Path(filename).suffix.lower()
         if ext in _IMAGE_EXTS:
-            cached_path = cache_image_from_bytes(payload, ext)
+            try:
+                cached_path = cache_image_from_bytes(payload, ext)
+            except ValueError:
+                logger.debug("Skipping non-image attachment %s (invalid magic bytes)", filename)
+                continue
             attachments.append({
                 "path": cached_path,
                 "filename": filename,