Fix flaky test_one_websocket_per_page after the offline test

[Archmonger]

Description

The periodic (nightly) Test/python-source run #35684228566 failed with:

FAILED tests/test_app/tests/test_components.py::ComponentTests::test_one_websocket_per_page
    AssertionError: Expected 1 unique WebSocket URL, got 2:
    {'ws://localhost:33203/reactpy/?path=%2F&qs=',
     'ws://localhost:42647/reactpy/?path=%2Foffline%2F&qs='}
CLIENT ERROR: WebSocket connection to 'ws://localhost:42647/reactpy/?path=%2Foffline%2F&qs='
    failed: ... net::ERR_CONNECTION_REFUSED

Root cause. The tests share a single browser page across the whole ComponentTests class, and they run in alphabetical order — so test_offline_component runs immediately before test_one_websocket_per_page.

test_offline_component calls self._server_process_1.terminate() while the page is still sitting on the /offline/ page. Tearing down the server severs that page's WebSocket, but the page's client (createReconnectingWebSocket in src/js/src/websocket.ts) keeps retrying the now-dead port on a backoff timer for the rest of the session. Because the shared page is never navigated away, that zombie reconnect loop keeps firing new WebSocket() against the dead port during subsequent tests.

test_one_websocket_per_page attaches a "websocket" listener and navigates to /. Any zombie reconnect attempt from the dead /offline/ page that lands inside that navigation window is recorded, so the test sees two unique URLs (the home page and the leaked /offline/ one) and fails. This is inherently probabilistic, which is why the identical commit passed on the previous nightly runs and only failed this one — exactly the signature of a flaky CI test.

Fix. Navigate the shared page to about:blank at the end of test_offline_component. Leaving the document cancels its pending reconnect timers, so no zombie client is left to leak WebSocket connections into later tests. This is the same class of fix as the earlier timing-related flakiness addressed in #299 and keeps the fix entirely within the test harness (no product-code change).

A minimal A/B reproduction (kill the offline server, then mimic test_one_websocket_per_page) fails 8/8 without the about:blank navigation and passes 0/8 with it. The full ComponentTests class and the whole single_db suite pass locally after the change.

Checklist

• Tests have been developed for bug fixes or new functionality. — No new assertion is needed; the existing test_one_websocket_per_page is the regression test, and it no longer flakes once the zombie reconnect is cleared.
• The changelog has been updated, if necessary. — Not applicable: this is a test-only CI-robustness change with no user-facing or API impact (consistent with the test-only flakiness fix in Fix flaky CI tests #299).
• Documentation has been updated, if necessary. — N/A.
• GitHub Issues closed by this PR have been linked. — No issue exists for this specific flake; it was first observed via the periodic run above.