fix(ci): revert .first() to .first in migration UI test_01_open_flow

Playwright Python locator.first is a property, not a method.
.first() introduced a TypeError — reverted to .first.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: lice-reis

tests/github-workflows/migration/test_ui_migration.py

@@ -59,7 +59,7 @@ def test_01_open_flow(self):
 
         # Verify the flow editor loaded (look for the canvas/reactflow area)
         canvas = self.page.locator(".react-flow, [data-testid='rf__wrapper']")
-        expect(canvas.first()).to_be_visible(timeout=20_000)
+        expect(canvas.first).to_be_visible(timeout=20_000)
 
         self.results["steps"]["open_flow"] = {"status": "pass"}
         self._save_results()