Read and display HTTP response body on WebSocket handshake failure

400/4xx responses include a body (Content-Length) with the actual error
message, which was previously swallowed. Now captured and shown in the
RuntimeError so the root cause is visible without --debug.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/portainer_cli/websocket_exec.rb

@@ -23,11 +23,7 @@ module PortainerCli
 
       socket = open_socket
 
-      handshake = perform_handshake(socket)
-      unless handshake.valid?
-        raise "WebSocket handshake failed: #{handshake.error}\nServer response:\n#{handshake.instance_variable_get(:@data)}"
-      end
-
+      perform_handshake(socket)
       run_io_loop(socket)
     ensure
       socket&.close rescue nil
@@ -59,15 +55,25 @@ module PortainerCli
       debug "Handshake request:\n#{handshake.to_s.chomp}"
       socket.write(handshake.to_s)
 
-      response = +''
+      header_bytes = +''
       loop do
         byte = socket.read(1)
         break if byte.nil?
-        response << byte
+        header_bytes << byte
         handshake << byte
         break if handshake.finished?
       end
-      debug "Handshake response:\n#{response.chomp}"
+      debug "Handshake response headers:\n#{header_bytes.chomp}"
+
+      unless handshake.valid?
+        # Try to read the response body for error details
+        body = ''
+        if (len = header_bytes.match(/Content-Length:\s*(\d+)/i)&.[](1)&.to_i) && len > 0
+          body = socket.read(len).to_s
+        end
+        raise "WebSocket handshake failed: #{handshake.error}\n" \
+              "Server response:\n#{header_bytes.chomp}\n#{body}"
+      end
 
       handshake
     end