docs: fix rawErrorResponseSchema description and React 19 StoreProvider pattern

[vishalranaut]

docs: fix rawErrorResponseSchema description and React 19 StoreProvider pattern

Closes #5257
Closes #5291

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What this PR does

Fixes two missing/outdated documentation issues in the RTK Query and Next.js guides.

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Change 1 — rawErrorResponseSchema missing description (createApi docs)

File: docs/rtk-query/api/createApi.mdx

The rawErrorResponseSchema property section existed as an anchor but had no body text, causing user confusion (see #4989).

Added a description that explains:

• It validates the error response before transformErrorResponse runs (against the raw server response)
• How it differs from errorResponseSchema, which validates after transformation
• When to use one vs. the other
• A short usage example

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Change 2 — Broken useRef store pattern in Next.js guide

File: docs/usage/nextjs.mdx

The existing StoreProvider example using useRef throws this error in React 19:

Error: Cannot access refs during render

Replaced it with the useState lazy initializer pattern:

const [store] = useState(() => makeStore())
return <Provider store={store}>{children}</Provider>

Also added a callout explaining why useRef breaks in React 19 concurrent/Strict Mode.

Verified against:

next: 15+
react: 19.x
@reduxjs/toolkit: ^2.x
react-redux: ^9.x

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Checklist

• I have read the CONTRIBUTING.md
• Docs only changes — no source code modified
• Tested locally with yarn docs to confirm both pages render correctly
• New StoreProvider example verified in a fresh Next.js 15 + React 19 project

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