Extend forum support from PR #10145:
- REST path (_send_discord): forum thread creation now uploads media
files as multipart attachments on the starter message in a single
call. Previously media files were silently dropped on the forum
path.
- Websocket media paths (_send_file_attachment, send_voice, send_image,
send_animation — covers send_image_file, send_video, send_document
transitively): forum channels now go through a new _forum_post_file
helper that creates a thread with the file as starter content,
instead of failing via channel.send(file=...) which forums reject.
- _send_to_forum chunk follow-up failures are collected into
raw_response['warnings'] so partial-send outcomes surface.
- Process-local probe cache (_DISCORD_CHANNEL_TYPE_PROBE_CACHE) avoids
GET /channels/{id} on every uncached send after the first.
- Dedup of TestSendDiscordMedia that the PR merge-resolution left
behind.
- Docs: Forum Channels section under website/docs/user-guide/messaging/discord.md.
Tests: 117 passed (22 new for forum+media, probe cache, warnings).
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