* fix(discord): accept inbound attachments and file-only messages
Empty content no longer drops messages that have attachments. Capture
CDN descriptors on inbound, download in receive_file via Bot auth, and
hand bytes through INBOUND_FILE_CONTENT_KEY like Slack/Telegram.
Closes#6856
* fix(discord): gate bot auth and share inbound file helpers
Address review on #6857: attach Bot Authorization only for Discord CDN
hosts, report Content-Encoding refusals as download failed, and hoist
filename/size helpers into app.channels.inbound_files (Telegram delegates
to the same module).
* fix(discord): sort telegram imports for ruff I001
CI lint-backend failed on unsorted imports in telegram.py after the
shared inbound_files helper was introduced.
* style: ruff-format discord inbound attachments test
* fix(discord): show the enforced attachment download limit
---------
Co-authored-by: Li-john1021 <Li-john1021@users.noreply.github.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
