fix(zod-openapi): preserve onError/notFound bindings set after basePath()

[tokiya-takai]

Issue: #2021

Summary

OpenAPIHono.basePath() dropped any .onError() / .notFound() registered on the returned instance
after the basePath() call. The handler silently had no effect, and the parent app's handler
ran instead. This surfaced while migrating a plain Hono app to OpenAPIHono: a route()-mounted
sub-app quietly changed which onError handled its thrown errors.

Reproduction

import { Hono } from 'hono'
import { OpenAPIHono } from '@hono/zod-openapi'

const sub = new OpenAPIHono().basePath('/v2')
sub.onError(() => new Response('from sub'))
sub.get('/boom', () => {
  throw new Error('boom')
})

const app = new Hono()
app.onError(() => new Response('from parent'))
app.route('/', sub)

const res = await app.request('/v2/boom')
console.log(await res.text()) // 'from parent' ← expected 'from sub'

Root cause

OpenAPIHono.basePath() rebuilt the instance by spreading the plain Hono clone returned by super.basePath():

return new OpenAPIHono({ ...(super.basePath(path) as any), defaultHook: this.defaultHook })

onError, notFound, request, and fetch are arrow-function instance fields, so they are own-enumerable properties permanently bound to the instance they were created on.
super.basePath() returns a throwaway clone, and spreading it copies those clone-bound methods
into the constructor options — where Object.assign(this, options) then overwrites the freshly constructed (and correctly bound) methods on the returned instance.

As a result, sub.onError(h) mutated errorHandler on the discarded clone, not on the returned
sub (sub.errorHandler stayed the built-in default). Route registration only appeared to work
because the returned instance and the clone share the same underlying router / routes. When
parent.route('/', sub) reads sub.errorHandler as a plain property, it still sees the default,
so it does not wrap the sub-app's handlers — and the parent's onError ends up handling the thrown error.

Fix

Construct a properly initialized OpenAPIHono (so its own arrow-field methods stay bound to it) and transplant only the routing state — router, routes, getPath, errorHandler, _basePath — from the clone, instead of spreading the clone into the constructor. Carrying errorHandler over preserves the "onError() before basePath()" ordering that core Hono's clone already captured.

Tests

• Should fire onError() registered after basePath() — regression guard for this issue (fails on main, passes with the fix).
• Should fire onError() registered before basePath() — guards the opposite ordering so the errorHandler transplant can't regress.
• notFound() set after basePath() targets the returned instance — a same-instance contract test. Note: route() propagates errorHandler but not notFoundHandler, so this documents the binding rather than acting as a regression guard (it passes with or without the fix; commented inline).

Verified locally: full @hono/zod-openapi suite passes (81 tests, type-check clean), ESLint clean, Prettier clean.

The author should do the following, if applicable

• Add tests
• Run tests
• pnpm changeset at the top of this repo and push the changeset
• Follow the contribution guide

[changeset-bot]

🦋 Changeset detected

Latest commit: b3b6619

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@hono/zod-openapi	Patch

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.08%. Comparing base (f03694d) to head (b3b6619).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2031      +/-   ##
==========================================
+ Coverage   92.07%   92.08%   +0.01%     
==========================================
  Files         115      115              
  Lines        4074     4081       +7     
  Branches     1063     1063              
==========================================
+ Hits         3751     3758       +7     
  Misses        287      287              
  Partials       36       36              

Flag	Coverage Δ
zod-openapi	95.12% <100.00%> (+0.21%)	⬆️

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[yusukebe]

LGTM!