test: add GatewayClient WebSocket unit tests (12 tests)

Cover core networking: connect/disconnect, challenge handshake,
event routing, request/response, error handling, timeout,
reconnect behavior, and credential updates.

Adds __APP_VERSION__ define to vitest.config.ts for test env.
Total test count: 95 → 107.

vitest.config.ts

@@ -1,6 +1,9 @@
 import { defineConfig } from 'vitest/config'
 
 export default defineConfig({
+  define: {
+    __APP_VERSION__: JSON.stringify('0.0.0-test'),
+  },
   test: {
     globals: true,
     environment: 'node',