Handle cases where the socket has already been assigned to the request

[jaburnette]

Bun 1.2.6 introduced a ton of changes to websocket handling. As part of those changes, sockets now seem to already be assigned to the response prior to fastify-websocket attempting to assign it. The specific error it results in is:

[08:29:49.659] WARN (95250): websocket upgrade failed
    err: {
      "type": "Error",
      "message": "Socket already assigned",
      "stack":
          Error: Socket already assigned
              at unknown
              at assignSocket (node:http:1275:31)
              at onUpgrade (/Users/justin/Projects/test-project/node_modules/@fastify/websocket/index.js:114:19)
              at emit (node:events:95:22)
              at onNodeHTTPRequest (node:http:558:24)
      "name": "Error",
      "code": "ERR_HTTP_SOCKET_ASSIGNED"
    }

All this change does is conditionally assign the socket to the response if it's not already assigned.

I've tested this with both node and bun and it seems to result in the correct behavior on both now.

Checklist

• run npm run test and npm run benchmark
• 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

[dancastillo]

Can you add a test for this use case