Add missing CORS headers defined in SockJS CORS config

Prior to this commit and following changes done in d27b5d0, the CORS
response headers would not be added for SockJS-related requests, even
though a CORS configuration had been applied to SockJS/WebSocket.
This was due to a missing case in our implementation: calling
`AbstractHandlerMapping#getHandlerInternal` can return a Handler
directly, but also a `HandlerExecutionChain` in some cases, as explained
in the Javadoc.

This commit ensures that, when checking for existing CORS configuration,
the `AbstractHandlerMapping` class also considers the
`HandlerExecutionChain` case and unwraps it to get the CORS
configuration from the actual Handler.

Fixes gh-23843

spring-webmvc/src/main/java/org/springframework/web/servlet/handler/AbstractHandlerMapping.java

@@ -485,6 +485,9 @@ public abstract class AbstractHandlerMapping extends WebApplicationObjectSupport
 	 * @since 5.2
 	 */
 	protected boolean hasCorsConfigurationSource(Object handler) {
+		if (handler instanceof HandlerExecutionChain) {
+			handler = ((HandlerExecutionChain) handler).getHandler();
+		}
 		return (handler instanceof CorsConfigurationSource || this.corsConfigurationSource != null);
 	}