fix(types): apply AppType<T> to top-level App props instead of pageProps

[Golenspade]

fix(types): apply AppType to top-level App props instead of pageProps

Problem
AppType incorrectly flowed T into AppPropsType's PageProps position, making T appear under props.pageProps rather than as top-level props (see #42846).

Fix

• Intersect T with top-level component props and AppInitialProps
• Keep AppPropsType's PageProps generic independent of T
• Use unknown for PageProps to avoid accidental property access through any

Validation

• Added a minimal tsc test (test/types/apptype-custom-props/app-type.test.ts)
  • Asserts props.foo exists at top-level
  • Asserts props.pageProps.foo does not exist (@ts-expect-error)
• Types-only change, no runtime impact

Closes: #42846

[ijjk]

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[Golenspade]

Additional context and validation details:

• Rationale: AppType<T> incorrectly funneled T into AppPropsType's PageProps, so T appeared under props.pageProps instead of as top-level props. This aligns with Parameter on AppType is used incorrectly #42846's repro and with the historical AppPropsType<Router, PageProps> shape.

• Fix summary: Intersect T with top-level component props and with AppInitialProps. Keep PageProps independent (set to unknown), preventing accidental property access via any.

• Minimal tsc test (added): test/types/apptype-custom-props/app-type.test.ts

  • Ensures props.foo exists on the top-level
  • Ensures props.pageProps.foo is flagged (@ts-expect-error)

• Local validation steps (types-only):

  1. Build only what's needed for d.ts: pnpm -F @next/font build then pnpm -F next types
  2. Run the focused test: ./node_modules/.bin/tsc -p test/types/apptype-custom-props/tsconfig.json

This is a type-only change and has no runtime impact.