fix(react-instantsearch): index added twice with Next.js app router

[aymeric-giraudet]

Summary

This should fix what @MikeIbberson talks about here : #6073 (comment)

The problem was index widgets would get added twice client-side in a Next.js App Router application.
This is due to how useLayoutEffect behaves on either React canary or an RSC context (not sure), before what we had was :

• index gets added on render because shouldSsr is true and the parent index doesn't contain it yet, making shouldAddWidgetEarly be true
• useLayoutEffect is called, shouldAddWidgetEarly is true so we don't add it again
• useLayoutEffect dispose function would be called and set the cleanupTimer
• shouldAddWidgetEarly is now false because parentIndex contains the child index
• useLayoutEffect called again, with shouldAddWidgetEarly being false this time, but it still skips adding the widget again because the cleanup timer is defined

In React 19 canary / Next.js, the cleanup function does not get called (maybe no strict mode or a different one), which means the last point fails.

Result

We were just using the wrong dependency in useEffect, we should never add the widget in an effect if shouldSsr is true, and that way the effect will run less often.
Only an index widget can have shouldSsr be true client-side

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Latest deployment of this branch, based on commit 9e6daa9:

Sandbox	Source
react-instantsearch-app	Configuration
example-react-instantsearch-default-theme	Configuration
example-react-instantsearch-next-app-dir-example	Configuration
example-vue-instantsearch-default-theme	Configuration

[dhayab]

While we're waiting until the API we use are stable to implement a suite of unit tests, this package could benefit from a simple e2e implementation in a future PR to check this scenario (similar to what we have for react-instantsearch-router-nextjs).