feat: introduce Bigtable Admin V2 clients for Selective GAPIC (#13173)

This PR introduces the modern `BigtableTableAdminClientV2` and
`BigtableInstanceAdminClientV2` clients in `java-bigtable` as
handwritten entry points for the Selective GAPIC hierarchy.

It is a migration of the original PR from the split repository
(googleapis/java-bigtable#2889) into the
`google-cloud-java` monorepo.

## Key Changes

### 1. Core V2 Clients & Callables
- **`BigtableInstanceAdminClientV2`**: Recreates the standardized admin
entry point extending `BaseBigtableInstanceAdminClient`.
- **`BigtableTableAdminClientV2`**: Recreates the administrative client
extending `BaseBigtableTableAdminClient`. It relocates manual wrappers
for Critical User Journeys (CUJs) like chained Long-Running Operations
(`awaitOptimizeRestoredTable`) and consistency token polling
(`waitForConsistency`).
- **`AwaitConsistencyCallableV2`**: Implements a completely decoupled,
lightweight utility callable for polling consistency tokens, ensuring
the V2 administrative client stack has no compile-time dependency on the
data client package.

### 2. Code Generation & POM Rules
- **`owlbot.py`**: Added a post-processing rule to remove the `final`
modifier from the `close()` method in `BaseBigtable*AdminClient` classes
to allow handwritten subclass overrides.
- **`pom.xml`**: Promoted the scope of
`grpc-google-cloud-bigtable-admin-v2` from `test` to compile scope to
support V2 admin gRPC descriptors and classes in production code.

---------

Co-authored-by: cloud-java-bot <cloud-java-bot@google.com>

Author: jinseopkim0

java-bigtable/google-cloud-bigtable/pom.xml

@@ -278,6 +278,7 @@
       <classifier>testlib</classifier>
       <scope>test</scope>
     </dependency>
+
     <dependency>
       <groupId>com.google.api.grpc</groupId>
       <artifactId>grpc-google-cloud-bigtable-admin-v2</artifactId>

java-bigtable/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/BaseBigtableInstanceAdminClient.java

@@ -5249,7 +5249,7 @@ public final void deleteMaterializedView(DeleteMaterializedViewRequest request)
   }
 
   @Override
-  public final void close() {
+  public void close() {
     stub.close();
   }
 

java-bigtable/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/BaseBigtableTableAdminClient.java

@@ -6414,7 +6414,7 @@ public final UnaryCallable<DeleteSchemaBundleRequest, Empty> deleteSchemaBundleC
   }
 
   @Override
-  public final void close() {
+  public void close() {
     stub.close();
   }
 

java-bigtable/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/BigtableInstanceAdminClientV2.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.cloud.bigtable.admin.v2;
+
+import com.google.cloud.bigtable.admin.v2.stub.BigtableInstanceAdminStub;
+import java.io.IOException;
+
+/**
+ * Modern Cloud Bigtable Instance Admin Client.
+ *
+ * <p>This client extends the {@link BaseBigtableInstanceAdminClient} to provide a simplified and
+ * enhanced API surface for managing Cloud Bigtable instances and clusters.
+ */
+public class BigtableInstanceAdminClientV2 extends BaseBigtableInstanceAdminClient {
+
+  protected BigtableInstanceAdminClientV2(BaseBigtableInstanceAdminSettings settings)
+      throws IOException {
+    super(settings);
+  }
+
+  protected BigtableInstanceAdminClientV2(BigtableInstanceAdminStub stub) {
+    super(stub);
+  }
+
+  /** Constructs an instance of BigtableInstanceAdminClientV2 with the given settings. */
+  public static final BigtableInstanceAdminClientV2 create(
+      BaseBigtableInstanceAdminSettings settings) throws IOException {
+    return new BigtableInstanceAdminClientV2(settings);
+  }
+}