fix: AppType generic parameter applies to props, not pageProps

[Jah-yee]

Fix: AppType generic parameter misuse (closes #42846)

Problem

Prop types defined by the generic parameter on AppType were incorrectly applied to MyApp's props.pageProps instead of MyApp's props.

Example from issue:

type MyInitialProps = { foo: string };
type MyAppType = AppType<MyInitialProps>;
const MyApp: MyAppType = ({ Component, foo, pageProps }) => {
  // foo should be in props, NOT pageProps.foo
  return <Component {...pageProps} />;
}

Error: Property 'foo' does not exist on type 'AppPropsType<any, MyInitialProps>'

Root Cause

In AppPropsType<Router, PageProps>, the PageProps type was nested inside AppInitialProps<PageProps> which wraps it as { pageProps: PageProps }. This caused custom props to end up inside pageProps rather than on the root props object.

Fix

Changed AppPropsType to merge PageProps directly onto the root props object:

- export type AppPropsType<...> = AppInitialProps<PageProps> & {
+ export type AppPropsType<...> = PageProps & {
+   pageProps: PageProps

This ensures:

1. Custom props (e.g., foo) are directly on props
2. pageProps field still exists with the same type for backwards compatibility

Breaking Changes

None — pageProps is still accessible, just also exposes custom props at the root level (which is the expected behavior based on how _app actually works at runtime).

[nextjs-bot]

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[nextjs-bot]

Allow CI Workflow Run

• approve CI run for commit: dd08dd0

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer