* fix(gateway): add media download retry to Mattermost, Slack, and base cache
Media downloads on Mattermost and Slack fail permanently on transient
errors (timeouts, 429 rate limits, 5xx server errors). Telegram and
WhatsApp already have retry logic, but these platforms had single-attempt
downloads with hardcoded 30s timeouts.
Changes:
- base.py cache_image_from_url: add retry with exponential backoff
(covers Signal and any platform using the shared cache helper)
- mattermost.py _send_media_url: retry on 429/5xx/timeout (3 attempts)
- slack.py _download_slack_file: retry on timeout/5xx (3 attempts)
- slack.py _download_slack_file_bytes: same retry pattern
* test: add tests for media download retry
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Co-authored-by: dieutx <dangtc94@gmail.com>
a2847ea7f0