fix(vercel): add void return type and ctx param for Next.js 15 App Router compatibility

[AliiiBenn]

Summary

The handle() function in hono/vercel adapter was incompatible with Next.js 15 App Router route handler types.

Two type errors were reported:

1. Return type was Response | Promise<Response> but Next.js 15 expects Response | void | Promise<Response | void>
2. Handler was missing the optional ctx parameter that Next.js 15 passes

Fix

• Added void to the return type union
• Added optional ctx?: unknown parameter to match Next.js 15 expectations

The runtime behavior is unchanged since app.fetch(req) always returns a Response (via notFoundHandler if needed).

Test plan

• Verify tsc --noEmit passes in a Next.js 15 App Router project using the documented setup

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.89%. Comparing base (1aa32fb) to head (e237eb6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4878   +/-   ##
=======================================
  Coverage   92.89%   92.89%           
=======================================
  Files         177      177           
  Lines       11797    11799    +2     
  Branches     3515     3514    -1     
=======================================
+ Hits        10959    10961    +2     
  Misses        837      837           
  Partials        1        1           

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