fix: correct socialExchange description and add null guards to example

Generated-By: mintlify-agent

Author: mintlify[bot]

mintlify/docs/sdk/auth.mdx

@@ -120,7 +120,7 @@ socialStart(provider: 'github' | 'google'): string
 
 ### socialExchange
 
-Exchange the `rtCode` received at your callback URL for a session.
+Exchange the `rtCode` received at your callback URL for a refresh token.
 
 ```typescript
 socialExchange(payload: SocialExchangePayload): Promise<SocialExchangeResponse>
@@ -134,6 +134,10 @@ const urlParams = new URLSearchParams(window.location.search);
 const rtCode = urlParams.get('rtCode');
 const token = new URLSearchParams(window.location.hash.slice(1)).get('token');
 
+if (!rtCode || !token) {
+  throw new Error('Missing OAuth callback parameters');
+}
+
 const { refreshToken } = await client.auth.socialExchange({ token, rtCode });
 ```