fix: Assistants conversations in db

[maxesse]

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Summary

This change saves the spec and iconURL for assistants in the db, as it happens for agents and base models. Prior to this fix, chats made with assistants would result in a generic sparkle icon on the chat list, and selecting one would put the modelspec picker to 'none selected'.

Change Type

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• Bug fix (non-breaking change which fixes an issue)
• [ ]

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