Feat(storage): add partial success bucket list (GoogleCloudPlatform#2180)

- Add `list_buckets_partial_success` snippet
- Handle `unreachable()` metadata in the sample
- Add system test to verify snippet execution

Author: thiyaguk09

storage/src/list_buckets_partial_success.php

@@ -0,0 +1,55 @@
+<?php
+/**
+ * Copyright 2026 Google Inc.
+ *
+ * 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
+ *
+ *     http://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.
+ */
+
+/**
+ * For instructions on how to run the full sample:
+ *
+ * @see https://github.com/GoogleCloudPlatform/php-docs-samples/tree/main/storage/README.md
+ */
+
+namespace Google\Cloud\Samples\Storage;
+
+# [START storage_list_buckets_partial_success]
+use Google\Cloud\Storage\StorageClient;
+
+/**
+ * Retrieves a list of buckets while gracefully handling regional downtime.
+ */
+function list_buckets_partial_success(): void
+{
+    $storage = new StorageClient();
+    $options = [ 'returnPartialSuccess' => true ];
+    $buckets = $storage->buckets($options);
+
+    // Check for unreachable locations first
+    // Note: unreachable() returns an array of strings for buckets in unavailable regions
+    if ($unreachable = $buckets->unreachable()) {
+        foreach ($unreachable as $location) {
+            printf('Unreachable Bucket: %s' . PHP_EOL, $location);
+        }
+    }
+
+    // Iterate through the buckets that were successfully retrieved
+    foreach ($buckets as $bucket) {
+        printf('Bucket: %s' . PHP_EOL, $bucket->name());
+    }
+}
+# [END storage_list_buckets_partial_success]
+
+// The following 2 lines are only needed to run the samples
+require_once __DIR__ . '/../../testing/sample_helpers.php';
+\Google\Cloud\Samples\execute_sample(__FILE__, __NAMESPACE__, $argv);

storage/test/storageTest.php

@@ -147,6 +147,13 @@ public function testListBuckets()
         $this->assertStringContainsString('Bucket:', $output);
     }
 
+    public function testListBucketsPartialSuccess()
+    {
+        $output = $this->runFunctionSnippet('list_buckets_partial_success');
+        $this->assertStringContainsString('Bucket:', $output);
+        $this->assertTrue(true);
+    }
+
     public function testListSoftDeletedBuckets()
     {
         $output = $this->runFunctionSnippet('list_soft_deleted_buckets');