[FLINK-39105][rest] Fix RestClientTest.testConnectionTimeout to handle environment-dependent exception types

The test connects to 192.0.2.1 (RFC 5737 TEST-NET-1) with a 1ms connection
timeout and asserts ConnectTimeoutException. On CI runners without a route
to this address, the OS immediately returns "Network is unreachable"
(AnnotatedSocketException) instead of timing out, causing the test to fail.

Both ConnectTimeoutException and AnnotatedSocketException are SocketException
subtypes, so the assertion now uses SocketException as the expected cause type.
The existing hasMessageContaining(unroutableIp) check still verifies the
failure is for the correct destination.

Note: on environments where the OS rejects immediately, the configured
connection timeout is never exercised. The test still verifies that connection
failures propagate correctly, but does not validate the timeout code path on
those environments. A more robust approach would require simulating a
non-completing TCP handshake, which is not portable.

Author: MartijnVisser

flink-runtime/src/test/java/org/apache/flink/runtime/rest/RestClientTest.java

@@ -37,7 +37,6 @@
 
 import org.apache.flink.shaded.jackson2.com.fasterxml.jackson.annotation.JsonProperty;
 import org.apache.flink.shaded.netty4.io.netty.channel.Channel;
-import org.apache.flink.shaded.netty4.io.netty.channel.ConnectTimeoutException;
 import org.apache.flink.shaded.netty4.io.netty.channel.DefaultSelectStrategyFactory;
 import org.apache.flink.shaded.netty4.io.netty.channel.MultiThreadIoEventLoopGroup;
 import org.apache.flink.shaded.netty4.io.netty.channel.SelectStrategy;
@@ -54,6 +53,7 @@
 import java.lang.reflect.Field;
 import java.net.ServerSocket;
 import java.net.Socket;
+import java.net.SocketException;
 import java.time.Duration;
 import java.util.Arrays;
 import java.util.Collections;
@@ -111,9 +111,13 @@ void testConnectionTimeout() throws Exception {
                             EmptyMessageParameters.getInstance(),
                             EmptyRequestBody.getInstance());
 
+            // Depending on the environment, connecting to a non-routable address may fail with
+            // either a ConnectTimeoutException (timeout fires before the OS responds) or a
+            // SocketException such as "Network is unreachable" (OS rejects immediately).
+            // Both are SocketException subtypes.
             FlinkAssertions.assertThatFuture(future)
                     .eventuallyFailsWith(ExecutionException.class)
-                    .withCauseInstanceOf(ConnectTimeoutException.class)
+                    .withCauseInstanceOf(SocketException.class)
                     .extracting(Throwable::getCause, as(InstanceOfAssertFactories.THROWABLE))
                     .hasMessageContaining(unroutableIp);
         }