* feat(agent): add PLATFORM_HINTS for matrix, mattermost, and feishu
These platform adapters fully support media delivery (send_image,
send_document, send_voice, send_video) but were missing from
PLATFORM_HINTS, leaving agents unaware of their platform context,
markdown rendering, and MEDIA: tag support.
Salvaged from PR #7370 by Rutimka — wecom excluded since main already
has a more detailed version.
Co-Authored-By: Marco Rutsch <marco@rutimka.de>
* test: add missing Markdown assertion for feishu platform hint
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Co-authored-by: Marco Rutsch <marco@rutimka.de>