core: the compat forward-compat test builds its feature list from the constants

A literal list went stale the moment attachment_sync was added (run 8513), the
same way pinned version numbers did at v2 — for a reason unrelated to what the
test checks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/src/sync/compat.rs

@@ -392,13 +392,20 @@ mod tests {
         // must not break the handshake.
         // Versions come from the constants, not literals: this test is about unknown
         // FIELDS, and pinning the numbers made it fail the moment the protocol moved
-        // to v2 — for a reason that has nothing to do with what it checks.
+        // to v2 — for a reason that has nothing to do with what it checks. The feature
+        // list likewise: a literal one went stale when `attachment_sync` was added.
+        let features: Vec<&str> = REQUIRED_FEATURES
+            .iter()
+            .chain(OPTIONAL_FEATURES.iter())
+            .copied()
+            .collect();
         let body = format!(
             r#"{{"site_name":"S","sync_protocol_version":{v},
                 "min_client_protocol_version":{v},
-                "sync_features":["notes","labels","attachments","tombstones","revisions"],
+                "sync_features":{f},
                 "some_future_field":{{"nested":true}}}}"#,
             v = CLIENT_PROTOCOL_VERSION,
+            f = serde_json::to_string(&features).expect("features"),
         );
         let info: ServerInfo = serde_json::from_str(&body).expect("unknown fields are ignored");
         assert_eq!(evaluate(&info), Compatibility::Ok);