Revert per-instance HTTP transport in BrowserStackClient

Customers may rely on the previous JVM-wide static HTTP_TRANSPORT
behaviour where setProxy on any client applied to all instances.
Restoring the static field to avoid breaking those setups.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Jimesh-browserstack

src/main/java/com/browserstack/client/BrowserStackClient.java

@@ -69,7 +69,7 @@ public Object parseAndClose(Reader reader, Type type) throws IOException {
             throw new IOException("Unsupported operation");
         }
     };
-    private HttpTransport httpTransport = new ApacheHttpTransport();
+    private static HttpTransport HTTP_TRANSPORT = new ApacheHttpTransport();
     protected final BrowserStackCache<String, Object> cacheMap;
 
     private HttpRequestFactory requestFactory;
@@ -105,8 +105,8 @@ public BrowserStackClient(String baseUrl, String username, String accessKey) {
         this.accessKey = accessKey.trim();
     }
 
-    HttpRequestFactory newRequestFactory() {
-        return httpTransport.createRequestFactory(httpRequest -> httpRequest.setParser(OBJECT_PARSER));
+    static HttpRequestFactory newRequestFactory() {
+        return HTTP_TRANSPORT.createRequestFactory(httpRequest -> httpRequest.setParser(OBJECT_PARSER));
     }
 
     static HttpRequest newRequest(final HttpRequestFactory requestFactory, final Method method, final GenericUrl url) throws BrowserStackException {
@@ -174,7 +174,7 @@ public void setProxy(final String proxyHost, final int proxyPort, final String p
         }
 
         final HttpClient client = clientBuilder.build();
-        this.httpTransport = new ApacheHttpTransport(client);
+        HTTP_TRANSPORT = new ApacheHttpTransport(client);
         this.requestFactory = newRequestFactory();
     }