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
0b143f2ea3