Do not include ETag in 304 responses matching Go's GetOrHeadHandler

Go sets ETag at L235 AFTER the If-Modified-Since and If-None-Match
304 return paths, so Go's 304 responses do not include the ETag header.
The Rust code was incorrectly including ETag in both 304 response paths.

seaweed-volume/src/server/handlers.rs

@@ -1006,9 +1006,7 @@ async fn get_or_head_handler_inner(
                             resp.headers_mut()
                                 .insert(header::LAST_MODIFIED, lm.parse().unwrap());
                         }
-                        // Go sets ETag before conditional checks, so 304 includes it
-                        resp.headers_mut()
-                            .insert(header::ETAG, etag.parse().unwrap());
+                        // Go sets ETag AFTER the 304 return paths (L235), so 304 does NOT include ETag
                         return resp;
                     }
                 }
@@ -1025,9 +1023,7 @@ async fn get_or_head_handler_inner(
                     resp.headers_mut()
                         .insert(header::LAST_MODIFIED, lm.parse().unwrap());
                 }
-                // Go sets ETag before conditional checks, so 304 includes it
-                resp.headers_mut()
-                    .insert(header::ETAG, etag.parse().unwrap());
+                // Go sets ETag AFTER the 304 return paths (L235), so 304 does NOT include ETag
                 return resp;
             }
         }