test(spanner): unflake EndpointLifecycleManagerTest requestEndpointRecreationSkippedWhenAddressNotActive (#14474)

In EndpointLifecycleManagerTest,
requestEndpointRecreationSkippedWhenAddressNotActive registers two
addresses concurrently ("server1", "server2"). EndpointLifecycleManager
dispatches background endpoint creation tasks across its 2-thread
scheduler, which concurrently invoked get() on the test double
FakeEndpointCache.

Because FakeEndpointCache stored endpoints in an unsynchronized
java.util.HashMap (unlike production GrpcChannelEndpointCache, which
uses ConcurrentHashMap), concurrent computeIfAbsent() calls on the
uninitialized map raced inside HashMap.resize(), silently dropping one
of the endpoints and causing awaitCondition to time out after 5s.

Fix:
- Use ConcurrentHashMap in FakeEndpointCache to match production channel
cache behavior.
- Mark mutable state fields in FakeEndpoint and FakeManagedChannel
volatile.
- Add regression test
concurrentBackgroundEndpointCreationDoesNotLoseEndpoints in
EndpointLifecycleManagerTest.

Author: sakthivelmanii

java-spanner/google-cloud-spanner/src/test/java/com/google/cloud/spanner/spi/v1/KeyRangeCacheTest.java

@@ -39,9 +39,9 @@
 import java.time.Instant;
 import java.time.ZoneId;
 import java.time.ZoneOffset;
-import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
 import javax.annotation.Nullable;
@@ -1925,9 +1925,9 @@ enum EndpointHealthState {
   // --- Test doubles ---
 
   static final class FakeEndpointCache implements ChannelEndpointCache {
-    private final Map<String, FakeEndpoint> endpoints = new HashMap<>();
+    private final Map<String, FakeEndpoint> endpoints = new ConcurrentHashMap<>();
     private final FakeEndpoint defaultEndpoint = new FakeEndpoint("default");
-    private boolean createOnGet = true;
+    private volatile boolean createOnGet = true;
 
     @Override
     public ChannelEndpoint defaultChannel() {
@@ -1977,7 +1977,7 @@ static final class FakeEndpoint implements ChannelEndpoint {
     private final String address;
     private final FakeManagedChannel channel = new FakeManagedChannel();
     private final AtomicInteger activeRequests = new AtomicInteger();
-    private EndpointHealthState state = EndpointHealthState.READY;
+    private volatile EndpointHealthState state = EndpointHealthState.READY;
 
     FakeEndpoint(String address) {
       this.address = address;
@@ -2042,7 +2042,7 @@ private static ConnectivityState toConnectivityState(EndpointHealthState healthS
   }
 
   private static final class FakeManagedChannel extends ManagedChannel {
-    private boolean shutdown = false;
+    private volatile boolean shutdown = false;
     private volatile ConnectivityState connectivityState = ConnectivityState.READY;
 
     void setConnectivityState(ConnectivityState state) {