integration tests: switch poll_for_navigation to Playwright wait_for_url

[BABTUNA]

Summary\nAfter #6378 migrated integration tests to Playwright, ests/integration/utils.py::poll_for_navigation still waits for URL changes via AppHarness.expect(...) (sleep-poll loop).\n\n## Problem\nThe custom polling loop is less reliable than Playwright's native navigation waiting and can miss/flake around redirect timing.\n\n## Proposed fix\nUpdate poll_for_navigation to use Playwright-native waiting:\n- capture prev_url before yielding\n- after the action, call page.wait_for_url(lambda url: url != prev_url, timeout=timeout * 1000)\n\nThis keeps current behavior (wait until URL changes) while using Playwright's event-driven waiter.\n\n## Acceptance criteria\n- poll_for_navigation no longer calls AppHarness.expect\n- it calls page.wait_for_url with an URL-change predicate\n- regression test covers the helper behavior\n

[linear]

ENG-9410