Agents with limited custom roles were receiving notifications (creation,
assignment, mentions, new messages, SLA) for conversations they couldn't
actually open. For example, an agent whose custom role only grants
`conversation_unassigned_manage` was getting notified about
conversations assigned to other agents.
Notifications now go through the same `ConversationPolicy#show?` check
that gates the conversation view itself, so an agent only gets notified
for conversations they're permitted to see. Administrators and agents
without custom roles are unaffected.
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Co-authored-by: Sojan Jose <sojan@pepalo.com>
Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>
