fix(slack): download files in Slack Connect channels

Slack Connect channels return file objects with file_access="check_file_info"
and no url_private_download field (see
https://docs.slack.dev/reference/objects/file-object/#slack_connect_files).
These stub objects must be resolved via files.info before download can
proceed. Without this the agent silently skips attachments posted in
Slack Connect channels.

Call files.info on every file whose file_access is check_file_info,
replace the stub with the full file object, and let the existing
download path continue. Warn and skip on files.info failures.

Closes #11095.

gateway/platforms/slack.py

@@ -1195,6 +1195,29 @@ class SlackAdapter(BasePlatformAdapter):
         media_types = []
         files = event.get("files", [])
         for f in files:
+            # Slack Connect channels return stub file objects with
+            # file_access="check_file_info" and no URL fields. We must
+            # call files.info to retrieve the full object (including url_private_download)
+            # before we can download it.
+            # https://docs.slack.dev/reference/objects/file-object/#slack_connect_files
+            if f.get("file_access") == "check_file_info":
+                file_id = f.get("id")
+                if not file_id:
+                    continue
+                try:
+                    info_resp = await self._get_client(channel_id).files_info(file=file_id)
+                    if info_resp.get("ok"):
+                        f = info_resp["file"]
+                    else:
+                        logger.warning(
+                            "[Slack] files.info failed for %s: %s",
+                            file_id, info_resp.get("error"),
+                        )
+                        continue
+                except Exception as e:
+                    logger.warning("[Slack] files.info error for %s: %s", file_id, e, exc_info=True)
+                    continue
+
             mimetype = f.get("mimetype", "unknown")
             url = f.get("url_private_download") or f.get("url_private", "")
             if mimetype.startswith("image/") and url: