On Node 24.15+, the `require` function passed to CJS modules loaded
via the ESM loader no longer has `.extensions` defined when the module
source comes through a custom loader (Yarn PnP's zip loader). This
causes `next build` to crash with a TypeError at module load time.

Add optional chaining on the top-level read and early-return guards
in registerHook/deregisterHook so the file loads safely when
require.extensions is undefined. No behavior change on Node versions
where require.extensions is defined.

Fixes vercel#92935

…ce tautological tests with real subprocess regression coverage

require-hook.ts:
- Add a `warnOnce` inside the `if (!require.extensions)` early-return so
  users with `require('./helper.ts')` in next.config.ts on Node 24.15+
  Yarn PnP get an actionable diagnostic instead of a downstream
  cryptic `TypeError`.
- Hoist the `oldJSHook` undefined check into the same early-return and
  drop the `oldJSHook!` non-null assertions. TypeScript can now narrow
  oldJSHook to a callable inside the closures, removing the runtime
  hazard of calling an undefined hook if the env transitioned.

require-hook.test.ts:
- The previous `Module._extensions = undefined` + `jest.isolateModules`
  simulation does not exercise the regression path. Jest's
  `_createRequireImplementation` unconditionally sets
  `moduleRequire.extensions = Object.create(null)` (a truthy empty
  object), so the SUT always sees a defined extensions object regardless
  of the outer process state. The "Node 24.15+ Yarn PnP" tests passed
  on canary too — they were tautological smoke tests.
- Replace them with subprocess tests that load the SUT inside a `vm`
  wrapper using a custom require whose `.extensions` is genuinely
  undefined, faithfully matching Yarn PnP's bridged require. Verified
  these tests fail on the unfixed source (TypeError reading '.js') and
  pass on the fixed source.
- Assert the new `warnOnce` fires from the registerHook subprocess test.
- Keep the existing jest-process tests as happy-path smoke coverage and
  document the limitation in a header comment.