fix(A.1 #8): make ResumeUnsupported test actually prove Range header was sent

Review feedback: the previous test would pass even if the Range-header
logic in download_file were deleted entirely, because File::create
truncates the stale .part regardless of which branch set
actually_resuming to false.

Tightens spawn_no_range_server to return HTTP 400 when the request
carries no Range header, and only return 200 + full body when Range is
present. A regression that stops sending Range now surfaces as a
download failure (empty body from 400, bytes != body assertion)
instead of silently passing through the truncation path.

crates/transcription/src/model_manager.rs

@@ -350,10 +350,16 @@ mod tests {
         addr
     }
 
-    /// A minimal HTTP server that always responds with 200 + the full
-    /// body, even when the request carries a `Range` header. Mirrors a
-    /// mirror / proxy that strips Range support, exercising the
-    /// ResumeUnsupported branch in `download_file`.
+    /// A minimal HTTP server that responds with 200 + full body **iff**
+    /// the request actually carries a `Range` header, and 400 otherwise.
+    /// This models a mirror / proxy that accepts Range requests but
+    /// refuses to honour them (returning a fresh full body), which is
+    /// exactly the ResumeUnsupported branch `download_file` needs to
+    /// handle. The 400-on-missing-Range behaviour is load-bearing for
+    /// the test: it turns "client never sent Range" into a download
+    /// failure, so deleting the resume-detection logic causes the test
+    /// to fail rather than pass coincidentally through File::create's
+    /// truncation semantics.
     async fn spawn_no_range_server(content: Vec<u8>) -> std::net::SocketAddr {
         let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
         let addr = listener.local_addr().unwrap();
@@ -361,7 +367,20 @@ mod tests {
         tokio::spawn(async move {
             let (mut socket, _) = listener.accept().await.unwrap();
             let mut buf = vec![0u8; 2048];
-            let _ = socket.read(&mut buf).await.unwrap();
+            let size = socket.read(&mut buf).await.unwrap();
+            let request = String::from_utf8_lossy(&buf[..size]).to_lowercase();
+
+            let saw_range_header = request
+                .lines()
+                .any(|line| line.trim_start().starts_with("range:"));
+
+            if !saw_range_header {
+                let response = "HTTP/1.1 400 Bad Request\r\n\
+                                Content-Length: 0\r\n\r\n";
+                socket.write_all(response.as_bytes()).await.unwrap();
+                return;
+            }
+
             let response = format!(
                 "HTTP/1.1 200 OK\r\n\
                  Content-Length: {}\r\n\r\n",