Add WebSocket Testing Support

[sdogruyol]

Summary

Implements #5

Adds two layers of WebSocket testing for Kemal apps: in-memory upgrade / handshake checks, and duplex tests that run real message exchange over a socket pair.

New helpers

• websocket_request_headers — Valid WebSocket upgrade headers; optional sec_key and Origin.
• get(path, …, websocket: true) — Merges upgrade headers and runs the request through SpecKemal.process_request. The ws block does not run (no upgrade_handler); use this for assertions on 101, Sec-WebSocket-Accept, or 400 / 426 on bad handshakes.
• connect_websocket — Real upgrade via a connected socket pair (UNIXSocket.pair on Unix, loopback TCP on Windows); invokes response.upgrade_handler like production, and yields a client HTTP::WebSocket for send/receive in the block.
• inner_connect_websocket — Advanced: access both the client WebSocket and the 101 HTTP::Client::Response inside the block.

Example

Assume your app prefixes server replies (so you can tell client send vs server send apart):

ws "/chat" do |socket, _env|
  socket.on_message { |msg| socket.send "server: #{msg}" }
end

1. Handshake only — fast; the ws body never runs:

get "/chat", websocket: true
response.status_code.should eq 101
response.headers["Upgrade"].should eq "websocket"

2. Real messages — runs the ws handler; assert the transformed reply, not the raw send:

connect_websocket "/chat" do |client|
  replies = Channel(String).new(1)
  client.on_message { |msg| replies.send msg }
  spawn { client.run }
  Fiber.yield

  client.send "hello"
  replies.receive.should eq "server: hello"
end