Function post_response_url 

pub async fn post_response_url(
    http: &Client,
    response_url: &str,
    body: Value,
) -> Result<(), String>

POST to a slash-command response_url. Stub for the deferred-response path (spec §2.5, §4.3) — not called anywhere in v1 scope; slash commands are explicitly out of scope for this PR. Kept so the module shape doesn’t need rework when they land.