fix(files_external): guard optional S3 key in storage id computation

S3 external mounts that authenticate via the AWS SDK default credential
chain (env vars, EC2 instance profile, ECS task role) carry no static
`key` param. AmazonS3::__construct() read $this->params['key']
unconditionally when deriving the storage id, emitting an "Undefined
array key" warning on every storage construction for such mounts.

Guard the access with `?? ''`. This aligns the id computation with
paramCredentialProvider(), which already treats `key` as optional and
falls through to the SDK default provider chain when it is absent. The
value only feeds an md5() hash, so keyless mounts keep a stable id.

Fixes #63564

Assisted-by: Kiro:claude-opus-4.8
Signed-off-by: Sebastian Cruz <default50@gmail.com>

Author: default50

apps/files_external/lib/Lib/Storage/AmazonS3.php

@@ -49,7 +49,7 @@ public function __construct(array $parameters) {
 		parent::__construct($parameters);
 		$this->parseParams($parameters);
 		// @todo: using `key` here may be problematic with different authentication methods and/or key rotation...
-		$this->id = 'amazon::external::' . md5($this->params['hostname'] . ':' . $this->params['bucket'] . ':' . $this->params['key']);
+		$this->id = 'amazon::external::' . md5($this->params['hostname'] . ':' . $this->params['bucket'] . ':' . ($this->params['key'] ?? ''));
 		$this->initCaches();
 		$this->mimeDetector = Server::get(IMimeTypeDetector::class);
 		/** @var ICacheFactory $cacheFactory */

apps/files_external/tests/Storage/Amazons3IdTest.php

@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Files_External\Tests\Storage;
+
+use OCA\Files_External\Lib\Storage\AmazonS3;
+use Test\TestCase;
+
+/**
+ * Regression test for the storage id computation in AmazonS3::__construct().
+ *
+ * Constructs the storage directly (no live S3 backend) to exercise only the id
+ * derivation, which runs unconditionally in the constructor.
+ */
+#[\PHPUnit\Framework\Attributes\Group('S3')]
+class Amazons3IdTest extends TestCase {
+	/**
+	 * Mounts authenticating via the AWS SDK default credential chain (env, EC2
+	 * instance profile, ECS task role) carry no static `key` param. The id must
+	 * still be derivable without emitting an "Undefined array key" warning
+	 * (PHPUnit fails the test on any emitted warning).
+	 */
+	public function testConstructWithoutKey(): void {
+		$storage = new AmazonS3(['bucket' => 'test-bucket']);
+		$this->assertNotEmpty($storage->getId());
+	}
+}