feat: Implement token storage via TokenCallback interface

- Add TokenCallback interface for token storage events
- Update OAuthConfig to include tokenCallback
- Update OAuthHandler to use callback for token events

Author: reeshika-h

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

@@ -9,6 +9,7 @@
 import java.util.List;
 
 import com.contentstack.cms.core.Util;
+import com.contentstack.cms.oauth.TokenCallback;
 
 /**
  * Configuration class for OAuth 2.0 authentication
@@ -27,6 +28,11 @@ public class OAuthConfig {
     private final String tokenEndpoint;
     private final String host;
 
+    /**
+     * Callback for token events
+     */
+    private final TokenCallback tokenCallback;
+
     /**
      * Validates the configuration
      *

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

@@ -193,6 +193,13 @@ public CompletableFuture<OAuthTokens> exchangeCodeForToken(String code) {
     private void _saveTokens(OAuthTokens tokens) {
         synchronized (tokenLock) {
             this.tokens = tokens;
+            if (config.getTokenCallback() != null) {
+                if (tokens != null) {
+                    config.getTokenCallback().onTokensUpdated(tokens);
+                } else {
+                    config.getTokenCallback().onTokensCleared();
+                }
+            }
         }
     }
 

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

@@ -0,0 +1,19 @@
+package com.contentstack.cms.oauth;
+
+import com.contentstack.cms.models.OAuthTokens;
+
+/**
+ * Callback interface for token events
+ */
+public interface TokenCallback {
+    /**
+     * Called when tokens are updated
+     * @param tokens The new tokens
+     */
+    void onTokensUpdated(OAuthTokens tokens);
+
+    /**
+     * Called when tokens are cleared
+     */
+    void onTokensCleared();
+}