fix(next): type AppType generic as app props

[bobu-putheeckal]

What?

Update AppType<P> so its generic parameter augments app props instead of pageProps.

Why?

The current type applies P to pageProps, which makes custom app-level props inferred from getInitialProps harder to type and does not match the apparent intent of the exported helper.

Closes #42846

How?

• Change AppType<P> to compose P with AppPropsType<any>.
• Extend the TypeScript e2e fixture to verify custom app props are available and not treated as pageProps.

Testing

• Focused tsc check for test/e2e/typescript/pages/_app.tsx
• git diff --check
• prettier --check on touched files