fix(tests): Reduce ~48s runtime of AI-generated slider bookmarking test

Part of #2308. `tests/playwright/ai_generated_apps/bookmark/input_slider/test_input_slider_express_bookmarking.py::test_slider_parameters[app-express.py]` takes **~48s on both chromium and webkit** (run [28639206793](https://github.com/posit-dev/py-shiny/actions/runs/28639206793)) — it single-handedly dominates its `playwright-ai` shard, where the next-slowest test is ~13s. Consistent cross-browser slowness points at the test itself: likely many slider drag interactions (each simulated with many small mouse moves) combined with bookmark/reload cycles. **Acceptance criteria:** 1. Identify which steps consume the time (drag simulation vs. reload/bookmark round-trips) 2. Reduce runtime substantially (e.g. fewer slider positions, set values programmatically where dragging isn't the behavior under test) while keeping bookmarking coverage