When a Slack message contains only files/audio (no text) and every file
download fails, `resolveSlackMedia` returns null and `rawBody` becomes
empty, causing `prepareSlackMessage` to silently drop the message.
Build a fallback placeholder from the original file names so the agent
still receives the message, matching the pattern already used in
`resolveSlackThreadHistory` for file-only thread entries.
Closes#25064