feat: add methodNotAllowed handler for 405 responses

[codewithkenzo]

Fixes #4633

Right now Hono returns 404 when you hit a route that exists but with the wrong HTTP method. That's not quite right per the HTTP spec - it should be 405 Method Not Allowed.

This PR adds a methodNotAllowed() handler (similar to how notFound() works) so you can customize the response, and includes the Allow header listing which methods actually work for that path.

What changed

• Added methodNotAllowed() method to customize the 405 response
• Router now distinguishes "path doesn't exist" (404) from "path exists but wrong method" (405)
• Default response includes the Allow header with valid methods
• Works with basePath, nested routers via app.route(), etc.

Usage

const app = new Hono()

// Optional - customize the response
app.methodNotAllowed((c, allowedMethods) => {
  return c.json({ 
    error: 'Method not allowed',
    allowed: allowedMethods 
  }, 405)
})

app.get('/users', (c) => c.text('List users'))
app.post('/users', (c) => c.text('Create user'))

// DELETE /users -> 405 with Allow: GET, POST
// GET /nonexistent -> 404 (unchanged)

Kept it simple and followed the existing patterns. Let me know if anything needs tweaking!

[yusukebe]

We can't accept this PR. Please read the issue at #4633 in full. Closing.