fix: ShimImpl uses shutdownAndAwait for userCallbackExecutor

ShimImpl registered its userCallbackExecutor with a bare shutdown() closer,
while Client.create's path uses shutdownAndAwait (which gives in-flight
listener.onClose tasks a 5-second drain window before shutdownNow). On
ShimImpl close, queued callbacks were abandoned mid-flight — fine for
quiescent shutdowns but a regression for fast-close patterns (test boundaries,
dynamic config reloads) where in-flight callbacks have not yet drained.

Promote Client.shutdownAndAwait from private-static to public-static so
ShimImpl (different package) can reuse the same shutdown semantics, and
update ShimImpl to call it.

Author: igorbernstein2

java-bigtable/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/api/Client.java

@@ -397,8 +397,10 @@ public T get() {
   }
 
   // Drain in-flight listener.onClose tasks before the executor is shut down; bound the wait at 5s
-  // so close() doesn't hang the caller on a pathological listener.
-  private static void shutdownAndAwait(ExecutorService exec) {
+  // so close() doesn't hang the caller on a pathological listener. Public so the compat
+  // ShimImpl (different package) can reuse the same shutdown semantics for the user-callback
+  // executor it owns.
+  public static void shutdownAndAwait(ExecutorService exec) {
     exec.shutdown();
     try {
       if (!exec.awaitTermination(5, TimeUnit.SECONDS)) {

java-bigtable/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/compat/ShimImpl.java

@@ -178,7 +178,8 @@ public static Shim create(
             Resource.createShared(metrics),
             Resource.createShared(configManager),
             Resource.createShared(bgExecutor),
-            Resource.createOwned(userCallbackExecutor, userCallbackExecutor::shutdown));
+            Resource.createOwned(
+                userCallbackExecutor, () -> Client.shutdownAndAwait(userCallbackExecutor)));
 
     return new ShimImpl(configManager, client);
   }