fix(test): add missing mod_time_unix_nanos field in mutation_hook test helper

The recent addition of ObjectDataCacheGetRequest.mod_time_unix_nanos
(backlog#1111 / ODC-06) missed the test helper in mutation_hook.rs,
causing a compile error under clippy --all-targets.

rustfs/src/app/object_data_cache/mutation_hook.rs

@@ -81,6 +81,7 @@ mod tests {
             version_id: None,
             etag: "etag",
             size: 5,
+            mod_time_unix_nanos: 0,
             body_variant: ObjectDataCacheBodyVariant::FullObjectPlainV1,
         }
     }