fix(vue): skip transitions for superseded pages

[hendriksenremco]

Issue number: resolves #31569

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What is the current behavior?

When a page in IonRouterOutlet mounts its <ion-page> after the route has already moved on to a newer page, its registerCallback still calls handlePageTransition(). That function reads the current route, so it runs the transition for the newer page, which has no ion-page yet. This logs the missing <ion-page> warning and throws TypeError: Cannot read properties of undefined (reading 'classList') in isViewVisible.

What is the new behavior?

• The registerCallback looks up the view item for the current route first. It only runs the transition when that item is the entering item, or when that item has no pending registerCallback.
• A superseded page that registers late no longer runs the transition for the newer page. The newer page runs its own transition once its ion-page registers.
• All other cases keep the existing behavior. That includes a newer page that has already registered, or a route that has no view item in this outlet.
• Adds a unit test in routing.spec.ts: two async pages, navigating to both before either loads, and loading the first one before the second.

Does this introduce a breaking change?

• Yes
• No

Other information

Testing:

• packages/vue: npm run lint, npm run typecheck and npm run build pass, with no diff after lint.
• Vue test app (vue3), npm run test:unit: 74 tests pass. The new test fails without the fix with the TypeError above.
• Cypress and Playwright e2e were not run locally.
• We run the same change in our production app (Nuxt + Capacitor + IonRouterOutlet) via a local yarn patch on @ionic/vue 8.8.5.

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