fix(notifications): Use the target id in guest conversion notifications

The rich object describing the converted-to account fell back to the
guest id when the target account could not be resolved, so a failed
conversion reported the guest twice instead of naming the account it was
supposed to become.

Signed-off-by: Git'Fellow <12234510+solracsf@users.noreply.github.com>

Author: solracsf

lib/Notifications/Notifier.php

@@ -49,7 +49,7 @@ private function getRichMessageParams(string $source, string $target): array {
 			],
 			'user' => [
 				'type' => $targetUser ? 'user' : 'highlight',
-				'id' => $targetUser?->getUID() ?? $source,
+				'id' => $targetUser?->getUID() ?? $target,
 				'name' => $targetUser?->getDisplayName() ?? $target,
 			],
 		];

tests/unit/Notifications/NotifierTest.php

@@ -0,0 +1,87 @@
+<?php
+
+declare(strict_types=1);
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Guests\Test\Unit\Notifications;
+
+use OCA\Guests\Notifications\Notifier;
+use OCP\IL10N;
+use OCP\IURLGenerator;
+use OCP\IUserManager;
+use OCP\L10N\IFactory;
+use OCP\Notification\INotification;
+use Test\TestCase;
+
+class NotifierTest extends TestCase {
+	private ?Notifier $notifier = null;
+
+	protected function setUp(): void {
+		parent::setUp();
+
+		$l10n = $this->createMock(IL10N::class);
+		$l10n->method('t')
+			->willReturnArgument(0);
+		$l10nFactory = $this->createMock(IFactory::class);
+		$l10nFactory->method('get')
+			->willReturn($l10n);
+
+		// Both accounts are gone by the time the notification is rendered.
+		$userManager = $this->createMock(IUserManager::class);
+		$userManager->method('get')
+			->willReturn(null);
+
+		$this->notifier = new Notifier(
+			$l10nFactory,
+			$this->createMock(IURLGenerator::class),
+			$userManager
+		);
+	}
+
+	public function testTransferDoneFallsBackToTheNotifiedIds(): void {
+		$parameters = $this->prepareTransferNotification('guest-transfer-done');
+
+		$this->assertSame('guest@example.tld', $parameters['guest']['id']);
+		$this->assertSame('new_account', $parameters['user']['id']);
+	}
+
+	public function testTransferFailFallsBackToTheNotifiedIds(): void {
+		$parameters = $this->prepareTransferNotification('guest-transfer-fail');
+
+		$this->assertSame('guest@example.tld', $parameters['guest']['id']);
+		$this->assertSame('new_account', $parameters['user']['id']);
+	}
+
+	/**
+	 * @return array<string, array<string, mixed>>
+	 */
+	private function prepareTransferNotification(string $subject): array {
+		$notification = $this->createMock(INotification::class);
+		$notification->method('getApp')
+			->willReturn('guests');
+		$notification->method('getSubject')
+			->willReturn($subject);
+		$notification->method('getSubjectParameters')
+			->willReturn([
+				'source' => 'guest@example.tld',
+				'target' => 'new_account',
+			]);
+		$notification->method('setRichSubject')
+			->willReturnSelf();
+
+		$parameters = [];
+		$notification->expects($this->once())
+			->method('setRichMessage')
+			->willReturnCallback(function (string $message, array $messageParameters) use (&$parameters, $notification): INotification {
+				$parameters = $messageParameters;
+				return $notification;
+			});
+
+		$this->notifier->prepare($notification, 'en');
+
+		return $parameters;
+	}
+}