fix: use credentials from existing client in custom OkHttpClient test

The test was using System.getenv() to read API credentials directly, but
the Gradle test JVM receives credentials as system properties (from
local.properties), not environment variables. Reuse the already-
initialized client's credentials, consistent with other tests.

Made-with: Cursor

Author: itsmeadi

src/test/java/io/getstream/StreamHTTPClientTest.java

@@ -149,9 +149,7 @@ void testCustomOkHttpClientPreservesConfig() {
             .readTimeout(45, TimeUnit.SECONDS)
             .build();
 
-    var sdkClient =
-        new StreamSDKClient(
-            System.getenv("STREAM_API_KEY"), System.getenv("STREAM_API_SECRET"), customHttp);
+    var sdkClient = new StreamSDKClient(client.getApiKey(), client.getApiSecret(), customHttp);
     OkHttpClient builtClient = sdkClient.getHttpClient().getHttpClient();
 
     assertSame(customPool, builtClient.connectionPool());