fix: capture promisified setImmediate separately (#88346)
### What?
Capture `setImmediate[promisify.custom]` at module initialization
instead of lazily accessing it when needed.
```
RangeError: Maximum call stack size exceeded
at patchedSetImmediatePromise (node_modules/.pnpm/next@16.1.1_@babel+core@7.28.5_react-dom@19.2.3_react@19.2.3__react@19.2.3/node_modules/next/src/server/node-environment-extensions/fast-set-immediate.external.ts:626:27)
at originalPromisify (node_modules/.pnpm/next@16.1.1_@babel+core@7.28.5_react-dom@19.2.3_react@19.2.3__react@19.2.3/node_modules/next/src/server/node-environment-extensions/fast-set-immediate.external.ts:627:12)
```
This was discovered upgrading from `16.0.10` to `16.1.0`
### Why?
When `util.promisify(setImmediate)` is called and `currentExecution ===
null`, the code was accessing `originalSetImmediate[promisify.custom]`
at call time. However, `promisify.custom` can get lost or return
`undefined` after the module patches `setImmediate`, causing
`promisify(setImmediate)` to fail. Specifically, when used with `jest`.
Reproduction:
https://github.com/jgeschwendt/nextjs-setimmediate-promisify-repro
- Failing:
https://github.com/jgeschwendt/nextjs-setimmediate-promisify-repro/actions/runs/20864170776
- Passing (with fix):
https://github.com/jgeschwendt/nextjs-setimmediate-promisify-repro/actions/runs/20864380068
-
https://github.com/jgeschwendt/nextjs-setimmediate-promisify-repro/pull/1
### How?
Move the capture of `originalSetImmediate[promisify.custom]` to module
initialization time (alongside `originalSetImmediate`,
`originalClearImmediate`, and `originalNextTick`), ensuring the original
promisified implementation is preserved before any patching occurs.
