Fix: Ensure .use() middleware works when path or prefix contains parameters

[MiccWan]

Summary

Fix .use() middleware being skipped when prefix or .use() path contains parameters in @koa/router v14 (see #202 ).

Problem

• In v14.0.0, @koa/router introduced a new option pathAsRegExp in Layer, which defaults to true for .use().
• When pathAsRegExp is true, .use() paths are not processed with path-to-regexp; instead, they are passed directly to new RegExp().
• As a result, if a .use() path or prefix contains parameters (e.g. /:id, /:version), the generated regex fails, and the .use() middleware is skipped entirely.

Minimal Reproduction

Example

import Router from '@koa/router';
import Koa from 'koa';

const app = new Koa();
const router1 = new Router();

router1
  .use('/:id', (ctx, next) => {
    // this will be skipped in @koa/router v14.0.0
    console.log('router1.use', ctx.params);
    return next();
  })
  .get('/:id', (ctx) => {
    console.log('router1.get', ctx.params);
    ctx.body = 'Hello World!\n';
  });

const router2 = new Router({ prefix: '/api/:version' });
router2
  .use((ctx, next) => {
    // this will be skipped in @koa/router v14.0.0
    console.log('router2.use', ctx.params);
    return next();
  })
  .get('/', (ctx) => {
    console.log('router2.get', ctx.params);
    ctx.body = 'Hello World!\n';
  });

app.use(router1.routes());
app.use(router2.routes());
app.listen(3000, async () => {
  await fetch('http://localhost:3000/123');
  await fetch('http://localhost:3000/api/123');
});

Steps to Reproduce

Run the script directly:

node index.js

Expected Behavior

Console should output:

router1.use { id: '123' }
router1.get { id: '123' }
router2.use { version: '123' }
router2.get { version: '123' }

Actual Behavior

Only the route handlers are triggered, .use() middlewares are skipped:

router1.get { id: '123' }
router2.get { version: '123' }

---

Changes

• Updated .use() registration:
  • Replaced the match-all regexp ([^/]*) with an empty string "".
  • This still matches all paths but avoids breaking parameter handling caused by pathAsRegExp.
• Check this.path is a string instead of checking if it is falsy in setPrefix and Layer.constructor.
• Tests:
  • Added 2 new tests to cover the parameter-related .use() middleware skipping issue.

Compatibility

• No breaking changes expected.
• All official usage patterns remain valid.

Checklist

• I have ensured my pull request is not behind the main or master branch of the original repository.
• I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• I have written a commit message that passes commitlint linting.
• I have ensured that my code changes pass linting tests.
• I have ensured that my code changes pass unit tests.
• I have described my pull request and the reasons for code changes along with context if necessary.