fix(auth): preserve refresh_token when refresh response omits it (#949)

Per RFC 6749 section 6 and OAuth 2.1: when the server does not issue a new
refresh_token on a refresh response, the client MUST keep the existing one.
AuthorizationManager::refresh_token() was persisting the response verbatim,
dropping the previous refresh_token and forcing full re-authorization on
the next refresh.

Match the fix from python-sdk#2270: preserve the existing refresh_token
when the response omits it, replace when the server rotates.

Fixes #921

Signed-off-by: SarthakB11 <sarthak.bhardwaj21b@iiitg.ac.in>

crates/rmcp/src/transport/auth.rs

@@ -1693,7 +1693,7 @@ impl AuthorizationManager {
         for scope in refresh_scopes {
             refresh_request = refresh_request.add_scope(Scope::new(scope));
         }
-        let token_result = refresh_request
+        let mut token_result = refresh_request
             .request_async(&OAuth2HttpClient {
                 client: self.http_client.as_ref(),
                 redirect_policy: self.refresh_redirect_policy,
@@ -1701,6 +1701,13 @@ impl AuthorizationManager {
             .await
             .map_err(|e| AuthError::TokenRefreshFailed(e.to_string()))?;
 
+        // RFC 6749 section 6: issuing a new refresh token on refresh is optional.
+        // When the response omits one, keep the existing refresh token rather than
+        // dropping it. When a new one is present, the response value is used as-is.
+        if token_result.refresh_token().is_none() {
+            token_result.set_refresh_token(Some(refresh_token_value));
+        }
+
         let granted_scopes: Vec<String> = match token_result.scopes() {
             Some(scopes) => scopes.iter().map(|s| s.to_string()).collect(),
             None => self.current_scopes.read().await.clone(),
@@ -5554,4 +5561,101 @@ mod tests {
             "scope should be absent when granted_scopes is empty, body: {body}"
         );
     }
+
+    #[tokio::test]
+    async fn refresh_token_preserves_existing_refresh_token_when_response_omits_it() {
+        use oauth2::TokenResponse;
+        // start_token_server returns a response without a refresh_token, matching
+        // an authorization server that does not rotate refresh tokens on refresh.
+        let (base_url, _captured) = start_token_server().await;
+
+        let mut manager = manager_with_metadata(Some(AuthorizationMetadata {
+            authorization_endpoint: format!("{}/authorize", base_url),
+            token_endpoint: format!("{}/token", base_url),
+            ..Default::default()
+        }))
+        .await;
+        manager.configure_client(test_client_config()).unwrap();
+
+        let stored = StoredCredentials {
+            client_id: "my-client".to_string(),
+            token_response: Some(make_token_response_with_refresh(
+                "old-token",
+                "my-refresh-token",
+            )),
+            granted_scopes: vec![],
+            token_received_at: Some(AuthorizationManager::now_epoch_secs()),
+        };
+        manager.credential_store.save(stored).await.unwrap();
+
+        let result = manager.refresh_token().await.unwrap();
+        assert_eq!(
+            result.refresh_token().map(|t| t.secret().as_str()),
+            Some("my-refresh-token"),
+            "returned response should keep the previous refresh token"
+        );
+
+        let reloaded = manager
+            .credential_store
+            .load()
+            .await
+            .unwrap()
+            .unwrap()
+            .token_response
+            .unwrap();
+        assert_eq!(
+            reloaded.refresh_token().map(|t| t.secret().as_str()),
+            Some("my-refresh-token"),
+            "stored credentials should keep the previous refresh token"
+        );
+    }
+
+    #[tokio::test]
+    async fn refresh_token_replaces_refresh_token_when_response_includes_new_one() {
+        use axum::{Router, body::Body, http::Response, routing::post};
+        use oauth2::TokenResponse;
+
+        let app = Router::new().route(
+            "/token",
+            post(|| async {
+                Response::builder()
+                    .status(200)
+                    .header("content-type", "application/json")
+                    .body(Body::from(
+                        r#"{"access_token":"new-token","token_type":"Bearer","expires_in":3600,"refresh_token":"rotated-refresh-token"}"#,
+                    ))
+                    .unwrap()
+            }),
+        );
+        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let addr = listener.local_addr().unwrap();
+        tokio::spawn(async move { axum::serve(listener, app).await.unwrap() });
+        let base_url = format!("http://{}", addr);
+
+        let mut manager = manager_with_metadata(Some(AuthorizationMetadata {
+            authorization_endpoint: format!("{}/authorize", base_url),
+            token_endpoint: format!("{}/token", base_url),
+            ..Default::default()
+        }))
+        .await;
+        manager.configure_client(test_client_config()).unwrap();
+
+        let stored = StoredCredentials {
+            client_id: "my-client".to_string(),
+            token_response: Some(make_token_response_with_refresh(
+                "old-token",
+                "my-refresh-token",
+            )),
+            granted_scopes: vec![],
+            token_received_at: Some(AuthorizationManager::now_epoch_secs()),
+        };
+        manager.credential_store.save(stored).await.unwrap();
+
+        let result = manager.refresh_token().await.unwrap();
+        assert_eq!(
+            result.refresh_token().map(|t| t.secret().as_str()),
+            Some("rotated-refresh-token"),
+            "a rotated refresh token from the response should replace the old one"
+        );
+    }
 }