refactor: Update OAuth configuration methods to enhance clarity and support for optional client secret

- Simplified setOAuth methods by removing the clientSecret parameter from the primary overloads.
- Added a new setOAuth method to accommodate an optional client secret while maintaining PKCE flow.
- Updated documentation to reflect changes in method signatures and clarify the use of client secret.

Author: reeshika-h

src/main/java/com/contentstack/cms/Contentstack.java

@@ -753,28 +753,39 @@ public Builder setTokenCallback(TokenCallback callback) {
         }
 
         /**
-         * Configures OAuth authentication. PKCE flow is automatically used when clientSecret is null or empty.
+         * Configures OAuth authentication with PKCE flow (no client secret)
          * @param appId Application ID
          * @param clientId Client ID
-         * @param clientSecret Client secret (optional). If null or empty, PKCE flow will be used
          * @param redirectUri Redirect URI
          * @return Builder instance
          */
-        public Builder setOAuth(String appId, String clientId, String clientSecret, String redirectUri) {
+        public Builder setOAuth(String appId, String clientId, String redirectUri) {
             // Use the builder's hostname (which defaults to Util.HOST if not set)
-            return setOAuth(appId, clientId, clientSecret, redirectUri, this.hostname);
+            return setOAuth(appId, clientId, redirectUri, this.hostname);
         }
 
         /**
-         * Configures OAuth authentication with a specific host. PKCE flow is automatically used when clientSecret is null or empty.
+         * Configures OAuth authentication with PKCE flow (no client secret) and specific host
          * @param appId Application ID
          * @param clientId Client ID
-         * @param clientSecret Client secret (optional). If null or empty, PKCE flow will be used
          * @param redirectUri Redirect URI
          * @param host API host (e.g. "api.contentstack.io", "eu-api.contentstack.com")
          * @return Builder instance
          */
-        public Builder setOAuth(String appId, String clientId, String clientSecret, String redirectUri, String host) {
+        public Builder setOAuth(String appId, String clientId, String redirectUri, String host) {
+            return setOAuth(appId, clientId, redirectUri, host, null);
+        }
+
+        /**
+         * Configures OAuth authentication with optional client secret. PKCE flow is used when clientSecret is not provided.
+         * @param appId Application ID
+         * @param clientId Client ID
+         * @param redirectUri Redirect URI
+         * @param host API host (e.g. "api.contentstack.io", "eu-api.contentstack.com")
+         * @param clientSecret Optional client secret. If not provided, PKCE flow will be used
+         * @return Builder instance
+         */
+        public Builder setOAuth(String appId, String clientId, String redirectUri, String host, String clientSecret) {
             OAuthConfig.OAuthConfigBuilder builder = OAuthConfig.builder()
                 .appId(appId)
                 .clientId(clientId)

src/test/java/com/contentstack/cms/oauth/OAuthTest.java

@@ -80,11 +80,11 @@ public void setup() {
 
         // Create Contentstack clients
         pkceClient = new Contentstack.Builder()
-                .setOAuth(TEST_APP_ID, TEST_CLIENT_ID, null, TEST_REDIRECT_URI)
+                .setOAuth(TEST_APP_ID, TEST_CLIENT_ID, TEST_REDIRECT_URI)
                 .build();
 
         clientSecretClient = new Contentstack.Builder()
-                .setOAuth(TEST_APP_ID, TEST_CLIENT_ID, TEST_CLIENT_SECRET, TEST_REDIRECT_URI)
+                .setOAuth(TEST_APP_ID, TEST_CLIENT_ID, TEST_REDIRECT_URI, Util.HOST, TEST_CLIENT_SECRET)
                 .build();
     }
 
@@ -113,7 +113,7 @@ public void testInvalidConfigurations() {
         // Test invalid app ID
         try {
             new Contentstack.Builder()
-                    .setOAuth("", TEST_CLIENT_ID, null, TEST_REDIRECT_URI)
+                    .setOAuth("", TEST_CLIENT_ID, TEST_REDIRECT_URI)
                     .build();
             fail("Should throw exception for empty app ID");
         } catch (IllegalArgumentException e) {
@@ -282,7 +282,7 @@ public void testHostStorage() {
 
         // Test host storage via Contentstack.Builder
         Contentstack client = new Contentstack.Builder()
-                .setOAuth(TEST_APP_ID, TEST_CLIENT_ID, TEST_CLIENT_SECRET, TEST_REDIRECT_URI, testHost)
+                .setOAuth(TEST_APP_ID, TEST_CLIENT_ID, TEST_REDIRECT_URI, testHost, TEST_CLIENT_SECRET)
                 .build();
 
         String authUrl = client.getOAuthAuthorizationUrl();
@@ -291,7 +291,7 @@ public void testHostStorage() {
 
         // Test host storage via PKCE builder
         client = new Contentstack.Builder()
-                .setOAuth(TEST_APP_ID, TEST_CLIENT_ID, null, TEST_REDIRECT_URI, testHost)
+                .setOAuth(TEST_APP_ID, TEST_CLIENT_ID, TEST_REDIRECT_URI, testHost)
                 .build();
 
         authUrl = client.getOAuthAuthorizationUrl();