Constant PAGE_CONTEXT_RESPONSE_ID 

const PAGE_CONTEXT_RESPONSE_ID: &str = "page-context";

Stands in for the Azure response id of a page context message, which we write ourselves and which therefore has no Azure response behind it. The not_null_for_llm_generated_messages check constraint demands one for every message that is not from the user; the initial assistant message of a conversation stands in for it the same way.