fix: accept method arrays on websocket routes

[lprnmns]

Fastify accepts method as a string or as an array, but the onRoute hook compares it to 'GET' directly. A websocket route declared with an array is rejected, even when it only lists GET:

fastify.route({
  method: ['GET'], // or ['GET', 'HEAD']
  url: '/echo',
  websocket: true,
  handler: (socket) => {}
})
// Error: websocket handler can only be declared in GET method

The hook now normalizes method to an array:

• only HEAD keeps returning early, as it does for the string form;
• any method other than GET or HEAD still throws the same error;
• ['GET'] and ['GET', 'HEAD'] are accepted and upgrade like method: 'GET'.

Tests cover both accepted arrays end to end with injectWS, and ['GET', 'POST'] still throwing. npm test passes, including 100% coverage and the type tests, and so does eslint.

Checklist

• run npm run test
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin and the Code of conduct