fix(mattermost): stop caching failed channel and user lookups (#137968)

fix(mattermost): stop caching failed channel and user lookups (#137968)

A failed channel or user lookup was cached as null for the full TTL
(5 minutes for channels, 10 minutes for users), so one transient API
error dropped every reaction and button click in that channel and broke
username allowlist matching for that sender until the entry expired.
Cache only resolved resources, as the Slack monitor already does, and
let the next event retry the lookup.