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/base.py

@@ -216,6 +216,23 @@ def get_image_cache_dir() -> Path:
     return IMAGE_CACHE_DIR
 
 
+def _looks_like_image(data: bytes) -> bool:
+    """Return True if *data* starts with a known image magic-byte sequence."""
+    if len(data) < 4:
+        return False
+    if data[:8] == b"\x89PNG\r\n\x1a\n":
+        return True
+    if data[:3] == b"\xff\xd8\xff":
+        return True
+    if data[:6] in (b"GIF87a", b"GIF89a"):
+        return True
+    if data[:2] == b"BM":
+        return True
+    if data[:4] == b"RIFF" and len(data) >= 12 and data[8:12] == b"WEBP":
+        return True
+    return False
+
+
 def cache_image_from_bytes(data: bytes, ext: str = ".jpg") -> str:
     """
     Save raw image bytes to the cache and return the absolute file path.
@@ -226,7 +243,17 @@ def cache_image_from_bytes(data: bytes, ext: str = ".jpg") -> str:
 
     Returns:
         Absolute path to the cached image file as a string.
+
+    Raises:
+        ValueError: If *data* does not look like a valid image (e.g. an HTML
+            error page returned by the upstream server).
     """
+    if not _looks_like_image(data):
+        snippet = data[:80].decode("utf-8", errors="replace")
+        raise ValueError(
+            f"Refusing to cache non-image data as {ext} "
+            f"(starts with: {snippet!r})"
+        )
     cache_dir = get_image_cache_dir()
     filename = f"img_{uuid.uuid4().hex[:12]}{ext}"
     filepath = cache_dir / filename