feat(oauth): add OAuth session caching to prevent multiple authentication attempts

ts/bunq.classes.apicontext.ts

@@ -162,4 +162,40 @@ export class BunqApiContext {
   public getBaseUrl(): string {
     return this.context.baseUrl;
   }
+
+  /**
+   * Check if the context has a valid session
+   */
+  public hasValidSession(): boolean {
+    return this.session && this.session.isSessionValid();
+  }
+
+  /**
+   * Initialize with existing OAuth session (skip installation/device/session creation)
+   */
+  public async initWithExistingSession(): Promise<void> {
+    // For OAuth tokens that already have a session, we just need to:
+    // 1. Use the OAuth token as the session token
+    // 2. Set OAuth mode for proper expiry handling
+    
+    this.context.sessionToken = this.options.apiKey;
+    
+    // Create session instance with existing token
+    this.session = new BunqSession(this.crypto, this.context);
+    this.session.setOAuthMode(true);
+    
+    // Try to get user info to validate the session
+    try {
+      // This will test if the session is valid
+      const testClient = this.session.getHttpClient();
+      const response = await testClient.get('/v1/user');
+      
+      if (response && response.Response) {
+        console.log('Successfully reused existing OAuth session');
+        await this.saveContext();
+      }
+    } catch (error) {
+      throw new Error(`Failed to reuse OAuth session: ${error.message}`);
+    }
+  }
 }