fix: multi-tab concurrency and metadata sync improvements

backend/test_browser_tabs.py

@@ -12,6 +12,8 @@ def service():
 def session(service):
     fake_context = AsyncMock()
     fake_page = AsyncMock()
+    # Playwright's page.on is synchronous
+    fake_page.on = MagicMock()
     fake_page.is_closed = MagicMock(return_value=False)
     fake_page.url = "https://initial.test"
     fake_page.title = AsyncMock(return_value="Initial Tab")
@@ -34,6 +36,7 @@ def session(service):
 @pytest.mark.asyncio
 async def test_tab_capture(service, session):
     new_page = AsyncMock()
+    new_page.on = MagicMock()
     new_page.is_closed = MagicMock(return_value=False)
     new_page.url = "https://new.test"
     new_page.title = AsyncMock(return_value="New Tab")
@@ -42,6 +45,9 @@ async def test_tab_capture(service, session):
     # Simulate page capture
     service._handle_new_page(session, new_page)
     
+    # Wait for the background registration task to finish
+    await asyncio.sleep(0.1)
+    
     assert len(session.tabs) == 2
     assert session.active_tab_id != "tab1"
     new_tab_id = session.active_tab_id