gatewayd: a turn's speaker, the username of the one person who wrote its burst

Co-Authored-By: ornith-1.5-35b-a3b <agent@wntrmute.dev>

crates/gatewayd/tests/support/fake_mm.rs

@@ -34,6 +34,8 @@ struct Inner {
     edits: Vec<(String, String)>,
     /// When set, `POST /posts` is refused with 400.
     refuse_posts: bool,
+    /// When set, `GET /users/<id>` (not `users/me`) answers 404.
+    refuse_users: bool,
 }
 
 pub struct FakeMm {
@@ -249,6 +251,10 @@ impl FakeMm {
     pub fn refuse_posts(&self, refuse: bool) {
         self.inner.lock().unwrap().refuse_posts = refuse;
     }
+
+    pub fn refuse_users(&self, refuse: bool) {
+        self.inner.lock().unwrap().refuse_users = refuse;
+    }
 }
 
 fn connection(mut stream: TcpStream, inner: &Mutex<Inner>, ws_tx: &mpsc::Sender<WsPeer>) {
@@ -367,6 +373,9 @@ fn rest(inner: &Mutex<Inner>, method: &str, path: &str, body: &[u8]) -> (u16, Va
             inner.edits.push((post_id.clone(), message.clone()));
             (200, post(&post_id, BOT, DM, "", &message, 1))
         }
+        ("GET", p) if p.starts_with("/api/v4/users/") && inner.refuse_users => {
+            (404, json!({"message": "not found"}))
+        }
         ("GET", p) if p.starts_with("/api/v4/users/") => {
             let user = p.trim_start_matches("/api/v4/users/");
             let name = if user == KYLE {