Fix error thrown in a mounted router routing to a pre-mount error handler (#239)

[emilioastarita]

Fixes #239.

Problem

An error thrown inside a router mounted with app.use(path, router) should be caught by an error handler declared after the mount (as express does), not one declared before it. ultimate-express routed it to the handler declared before the mount:

// router.js
const router = require("ultimate-express").Router();
router.post("/path", (req, res) => { throw new Error("Whoops"); });
module.exports = router;

// index.js
const app = express();
app.use(function jsonErrorHandler(err, req, res, next) { if (err) return res.send("JSON error"); next(); });
app.use("/route", require("./router"));
app.use(function errorHandler(err, req, res, next) { if (err) return res.json(err.toString()); next(); });
app.listen(3000);
// POST /route/path -> express: "Error: Whoops"  |  ultimate-express: "JSON error"

The reporters noted it was flaky and only happened when the router was imported from another module. Both observations follow from the cause below; verified against express 4 and express 5.2.x.

Cause

The route optimizer replays a mounted router's leaf route inline in the parent's uWS fast-path chain. Normal dispatch attributes an error propagating out of a sub-router to the mount (req._errorKey = <mount route key> after the sub-router returns), but the optimized chain never did this, so req._errorKey kept the leaf route's key.

A router's routes get a lower routeKey than the parent's own middlewares when the router is constructed before them — which is exactly what happens when it is required from another module. An error handler is selected when route.routeKey >= req._errorKey, so a pre-mount handler (key higher than the leaf's) wrongly qualified. "Flaky / only from another module" is this ordering; "only sometimes" was the old ~100ms optimizer window (now compiled at listen, so it is consistent).

Fix

When the optimized chain is exhausted and dispatch falls back to normal routing, attribute a pending error to the mount's routeKey (mirroring normal dispatch), and make skipUntil the mount boundary rather than the router's leaf so fallthrough resumes after the mount instead of re-running the parent middlewares that precede it. Both are gated on the chain actually crossing a mounted router (the keepMount marker), so plain top-level optimized routes are unaffected.

Test

New tests/tests/routers/mounted-router-error-handler-order.js: a router defined before the parent's handlers (reproducing the lower-routeKey condition in a single file), with an error-throwing route and a normal route. Fails on main, passes with the fix, output identical to express 4 and express 5. Full suite passes.