[hotfix] prevent app from crashing when resending messages after safety number retrust

TK-TODO: write a proper merge request when we are less pressed for time to get this out the door!

• SYMPTOM: when trying to send an attachment on a channel in which subscribers have recently reinstalled signal, signald crashes with a deserialization error
• CAUSE: the request message provided by signald is similarly hybrid-formatted as the one we receive on rate limiting errors (the data envelop looks like an inbound message, but the attachments look like an outbound message). however, unlike in the rate limit error retry logic, we were failing to properly convert these request fields before attempting a resend. thus, when signald tried to resend, it hit an object instead of a string when trying to deserialize the attachment.preview field and crashed
• FIX: add new parse logic to properly convert either inbound messages or these hybrid retry request messages into a proper OutBoundMessage