fix: Update OAuth implementation 5

Author: reeshika-h

src/main/java/com/contentstack/cms/models/OAuthConfig.java

@@ -94,7 +94,7 @@ public String getTokenEndpoint() {
             .replaceAll("^dev\\d+", "dev")  // Replace dev1, dev2, etc. with dev
             .replace("io", "com");
 
-        return "https://" + hostname + "/apps/oauth/token";
+        return "https://" + hostname + "/token";
     }
 
     /**

src/main/java/com/contentstack/cms/oauth/OAuthHandler.java

@@ -65,7 +65,8 @@ public OAuthHandler(OkHttpClient httpClient, OAuthConfig config) {
      */
     private Request.Builder _getHeaders() {
         return new Request.Builder()
-            .header("Content-Type", "application/x-www-form-urlencoded");
+            .header("Content-Type", "application/x-www-form-urlencoded")
+            .header("authorization", "Bearer " + (tokens != null ? tokens.getAccessToken() : ""));
     }
 
     /**
@@ -150,7 +151,8 @@ public CompletableFuture<OAuthTokens> exchangeCodeForToken(String code) {
                     .add("grant_type", "authorization_code")
                     .add("code", code)
                     .add("redirect_uri", config.getRedirectUri())
-                    .add("client_id", config.getClientId());
+                    .add("client_id", config.getClientId())
+                    .add("app_id", config.getAppId());
 
                 // Choose between client_secret and code_verifier like JS SDK
                 if (config.getClientSecret() != null) {