Function replayable_input_message 

pub(super) fn replayable_input_message(
    message: ChatbotConversationMessage,
) -> ChatbotResult<Option<APIInputMessage>>

One message a round just stored, as the next round should carry it, or None for a reasoning item Azure would reject: without an encrypted_content payload it cannot be resolved once responses are not stored Azure-side.

The in-turn counterpart of replayable_input_messages, which applies the same rule plus the ordering one that only a stored conversation can be checked against.