[TASK] Add canary test to detect upstream fix

Add a test that expects tempnam() to trigger E_NOTICE when the
directory doesn't exist. When this test FAILS, it indicates that
PHP's behavior has changed or upstream has fixed the issue, and
the DecoratingPlantumlBinaryRenderer workaround may no longer
be needed.

This helps track when we can safely remove this workaround.

Relates: TYPO3-Documentation#1099

Author: CybotTM

packages/typo3-docs-theme/tests/unit/Renderer/DecoratingPlantumlBinaryRendererTest.php

@@ -10,14 +10,61 @@
 use PHPUnit\Framework\TestCase;
 use T3Docs\Typo3DocsTheme\Renderer\DecoratingPlantumlBinaryRenderer;
 
+use function file_exists;
 use function is_dir;
 use function rmdir;
+use function set_error_handler;
 use function sys_get_temp_dir;
+use function tempnam;
+use function unlink;
 
 final class DecoratingPlantumlBinaryRendererTest extends TestCase
 {
     private const TEMP_SUBDIRECTORY = '/phpdocumentor';
 
+    /**
+     * Canary test: Verifies that tempnam() still triggers E_NOTICE when directory doesn't exist.
+     *
+     * This test documents the upstream bug that DecoratingPlantumlBinaryRenderer works around.
+     * When this test FAILS, the upstream library (phpDocumentor/guides-graphs) has likely
+     * fixed the issue, and this decorator may no longer be needed.
+     *
+     * @see https://github.com/phpDocumentor/guides-graphs/issues/1
+     * @see https://github.com/TYPO3-Documentation/render-guides/pull/1099
+     */
+    #[Test]
+    public function tempnamStillTriggersNoticeWhenDirectoryMissing(): void
+    {
+        // Use a unique non-existent directory to avoid interference
+        $nonExistentDir = sys_get_temp_dir() . '/phpdocumentor_canary_' . uniqid();
+
+        $noticeTriggered = false;
+        $previousHandler = set_error_handler(static function (int $errno, string $errstr) use (&$noticeTriggered): bool {
+            if ($errno === E_NOTICE && str_contains($errstr, 'tempnam()')) {
+                $noticeTriggered = true;
+            }
+            return false; // Let PHP handle it normally
+        });
+
+        try {
+            $tempFile = @tempnam($nonExistentDir, 'canary_');
+
+            // Clean up the temp file if it was created (in system temp dir)
+            if ($tempFile !== false && file_exists($tempFile)) {
+                unlink($tempFile);
+            }
+        } finally {
+            set_error_handler($previousHandler);
+        }
+
+        self::assertTrue(
+            $noticeTriggered,
+            'Expected tempnam() to trigger E_NOTICE when directory does not exist. '
+            . 'If this test fails, the upstream issue may be fixed and DecoratingPlantumlBinaryRenderer '
+            . 'might no longer be needed. Check: https://github.com/phpDocumentor/guides-graphs/issues/1'
+        );
+    }
+
     #[Test]
     public function renderCreatesTempDirectoryWhenMissing(): void
     {