Avoid GitHub API rate limits for create-next-app examples (#97612)
### What?
Use GitHub's raw content endpoint to validate the `package.json` for
create-next-app examples supplied as repository URLs.
### Why?
The previous validation used GitHub's unauthenticated Contents API.
Parallel create-next-app tests can exhaust the shared runner IP's low
API quota, after which valid examples are reported as missing before
download or package-manager installation begins.
This surfaced as `EPERM` because execa labels a child exit code of `1`
with Node's matching errno name; the underlying create-next-app output
showed the repository lookup failure.
### How?
Probe the example's `package.json` directly on
`raw.githubusercontent.com`. This preserves the existing existence check
while removing the rate-limited API request from URL-based example
creation. The actual archive download continues to use
`codeload.github.com` as before.
### Verification
- `IS_WEBPACK_TEST=1 NEXT_TEST_MODE=start pnpm test-start
test/production/create-next-app/package-manager/yarn.test.ts`
- `pnpm build-all`
- `pnpm --filter create-next-app build`
- `pnpm types`
<!-- NEXT_JS_LLM -->
Co-authored-by: vercel-fleet-prod[bot] <318278635+vercel-fleet-prod[bot]@users.noreply.github.com>
Co-authored-by: Tobias Koppers <1365881+sokra@users.noreply.github.com>