[6.x] Fix toasts being replayed on partial reloads

[heidkaemper]

This PR fixes a problem where toast messages on Inertia CP pages are replayed on every partial reload. We ran into this on a custom page that polls a queue via usePoll(5000, { only: [...] }) after a form.post() redirect. The success toast reappeared every 5 seconds until navigating away.

The problem: _toasts is shared as a regular Inertia prop, but it is flash data. Partial responses only contain the requested props, and Inertia merges them into the existing page props. So a stale _toasts survives every partial reload, and the success listener in Toasts.js re-displays it each time.

To reproduce the problem: Flash a toast on any CP page that uses usePoll(…, { only: [...] }).
See "Reproduction Code" down below.

The fix removes the prop from the page props once the toasts have been displayed, so a later partial reload has nothing left to replay.

An alternative approach would be to change toasts logic to Inertia's flash data (Inertia::flash()). But that requires inertia-laravel >= 2.0.16 and touches the initial page load path (displayInitialToasts()). So it felt a bit too big of a change for this bugfix.

---

Reproduction Code

// routes/cp.php
Route::get('toast-repro', fn () => Inertia::render('toast-repro/Index', ['time' => now()->toTimeString()]));
Route::post('toast-repro', fn () => redirect('/cp/toast-repro')->with('success', 'Flashed once'));

<!-- resources/js/cp/pages/toast-repro/Index.vue -->
<script setup>
import { router, usePoll } from '@statamic/cms/inertia'

defineProps({ time: String })

usePoll(3000, { only: ['time'] })
</script>

<template>
    <div>
        <p>{{ time }}</p>
        <button @click="router.post('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/cp/toast-repro')">Flash a toast</button>
    </div>
</template>

// resources/js/cp/cp.js
import Index from './pages/toast-repro/Index.vue'

Statamic.$inertia.register('toast-repro/Index', Index)

Click the button: expected is one toast, actual is a toast every 3 seconds