remove fixme. Add unit tests

Author: stevensJourney

packages/powersync_core/lib/src/sync/streaming_sync.dart

@@ -775,27 +775,7 @@ final class _ActiveRustStreamingIteration {
             },
             line);
       case EstablishSyncStream(:final request):
-        // FIXME
-        // Merge app_metadata from options into the instruction request
-        // since the Rust instruction does not yet merge app_metadata from the supplied options.
-        // Rust client values will override options values if present.
-        final mergedRequest = Map<String, Object?>.from(request);
-        final appMetadata = sync.options.appMetadata;
-        if (appMetadata.isNotEmpty) {
-          final existingMetadata = request['app_metadata'];
-          if (existingMetadata is Map) {
-            // Merge: start with options, then let Rust override
-            mergedRequest['app_metadata'] = {
-              ...appMetadata,
-              ...existingMetadata.cast<String, String>(),
-            };
-          } else {
-            // No existing metadata, just use options
-            mergedRequest['app_metadata'] = appMetadata;
-          }
-        }
-        _completedStream
-            .complete(_handleLines(EstablishSyncStream(mergedRequest)));
+        _completedStream.complete(_handleLines(EstablishSyncStream(request)));
       case UpdateSyncStatus(:final status):
         sync._state.updateStatus((m) => m.applyFromCore(status));
       case FetchCredentials(:final didExpire):

packages/powersync_core/test/sync/stream_test.dart

@@ -4,7 +4,6 @@ import 'dart:convert';
 import 'package:async/async.dart';
 import 'package:logging/logging.dart';
 import 'package:powersync_core/powersync_core.dart';
-
 import 'package:test/test.dart';
 
 import '../server/sync_server/in_memory_sync_server.dart';
@@ -260,4 +259,40 @@ void main() {
     );
     a.unsubscribe();
   });
+
+  test('passes app metadata to the server (rust)', () async {
+    options = SyncOptions(
+      syncImplementation: SyncClientImplementation.rust,
+      appMetadata: {'foo': 'bar'},
+    );
+
+    await waitForConnection();
+
+    final request = await syncService.waitForListener;
+    expect(
+      json.decode(await request.readAsString()),
+      containsPair(
+        'app_metadata',
+        containsPair('foo', 'bar'),
+      ),
+    );
+  });
+
+  test('passes app metadata to the server (legacy sync client)', () async {
+    options = SyncOptions(
+      syncImplementation: SyncClientImplementation.dart,
+      appMetadata: {'foo': 'bar'},
+    );
+
+    await waitForConnection();
+
+    final request = await syncService.waitForListener;
+    expect(
+      json.decode(await request.readAsString()),
+      containsPair(
+        'app_metadata',
+        containsPair('foo', 'bar'),
+      ),
+    );
+  });
 }