Preserve PathBase in SPA back/forward flows by using server canonical URL for initial request state

[Copilot]

From 4.3.9 onward, SPA back navigation could leave client request state pointing to a PathBase-stripped URL, causing subsequent SPA-triggered requests/navigation to target wrong paths (e.g. /spa-page2 instead of /subpath/spa-page2).
This change keeps SPA request state aligned with the server-canonical route URL returned by navigation responses.

• SPA URL state handling

  • Update SPA navigation state refresh to prefer response.result.url (server canonical URL) over the client-normalized URL.
  • This keeps getInitialUrl() PathBase-aware after SPA navigation/back-forward restoration.

• Regression coverage

  • Add a focused script-level test that simulates SPA navigation with a server response URL containing a base path and asserts that getInitialUrl() is updated to that server URL.

• Behavioral impact

  • Subsequent postbacks/static commands continue using the correct PathBase-prefixed endpoint after browser back/forward in SPA scenarios.

// before
updateInitialUrl(url);

// after
updateInitialUrl(response.result.url ?? url);