Add size_hint to TrackedBody so throttled downloads get Content-Length

TrackedBody (used for download throttling) did not implement
size_hint(), causing HTTP/1.1 to fall back to chunked transfer
encoding instead of setting Content-Length. Go always sets
Content-Length explicitly for non-range responses.

seaweed-volume/src/server/handlers.rs

@@ -61,6 +61,10 @@ impl http_body::Body for TrackedBody {
         let data = std::mem::take(&mut self.data);
         std::task::Poll::Ready(Some(Ok(http_body::Frame::data(bytes::Bytes::from(data)))))
     }
+
+    fn size_hint(&self) -> http_body::SizeHint {
+        http_body::SizeHint::with_exact(self.data.len() as u64)
+    }
 }
 
 impl Drop for TrackedBody {