[TASK] Add tests for invalid guides.xml error handling

Adds integration tests that verify:
- Invalid guides.xml shows helpful error message instead of PHP Fatal error
- XSD validation error includes line number
- Valid guides.xml files still render successfully

Author: CybotTM

tests/Integration/InvalidGuidesXmlTest.php

@@ -0,0 +1,97 @@
+<?php
+
+declare(strict_types=1);
+
+namespace T3Docs\Typo3DocsTheme\Integration;
+
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Process\Process;
+
+/**
+ * Tests error handling for invalid guides.xml configurations.
+ */
+final class InvalidGuidesXmlTest extends TestCase
+{
+    private const FIXTURES_PATH = __DIR__ . '/tests/invalid-guides-xml/input';
+
+    public function testInvalidGuidesXmlShowsHelpfulErrorMessage(): void
+    {
+        $binPath = dirname(__DIR__, 2) . '/bin/guides';
+
+        $process = new Process([
+            'php',
+            $binPath,
+            'run',
+            '--config=' . self::FIXTURES_PATH,
+            self::FIXTURES_PATH,
+        ]);
+
+        $process->run();
+
+        // Should fail with exit code 1, not crash with fatal error
+        self::assertSame(1, $process->getExitCode(), 'Expected exit code 1 for invalid guides.xml');
+
+        $stderr = $process->getErrorOutput();
+
+        // Should contain helpful error message, not PHP fatal error
+        self::assertStringContainsString('Invalid guides.xml configuration', $stderr);
+        self::assertStringNotContainsString('PHP Fatal error', $stderr);
+        self::assertStringNotContainsString('Stack trace', $stderr);
+
+        // Should reference documentation
+        self::assertStringContainsString('https://docs.typo3.org', $stderr);
+    }
+
+    public function testInvalidGuidesXmlShowsLineNumber(): void
+    {
+        $binPath = dirname(__DIR__, 2) . '/bin/guides';
+
+        $process = new Process([
+            'php',
+            $binPath,
+            'run',
+            '--config=' . self::FIXTURES_PATH,
+            self::FIXTURES_PATH,
+        ]);
+
+        $process->run();
+
+        $stderr = $process->getErrorOutput();
+
+        // Should show XSD validation error with line number
+        self::assertStringContainsString('Line 3', $stderr);
+        self::assertStringContainsString('theme', $stderr);
+    }
+
+    public function testValidGuidesXmlRendersSuccessfully(): void
+    {
+        $binPath = dirname(__DIR__, 2) . '/bin/guides';
+        $validFixturePath = __DIR__ . '/tests/getting-started/input';
+
+        // Skip if fixture doesn't exist
+        if (!is_dir($validFixturePath)) {
+            self::markTestSkipped('Valid fixture not available');
+        }
+
+        $outputPath = sys_get_temp_dir() . '/render-guides-test-' . uniqid();
+
+        $process = new Process([
+            'php',
+            $binPath,
+            'run',
+            '--config=' . $validFixturePath,
+            '--output=' . $outputPath,
+            $validFixturePath,
+        ]);
+
+        $process->run();
+
+        // Clean up
+        if (is_dir($outputPath)) {
+            system('rm -rf ' . escapeshellarg($outputPath));
+        }
+
+        // Should succeed
+        self::assertSame(0, $process->getExitCode(), 'Expected exit code 0 for valid guides.xml. Error: ' . $process->getErrorOutput());
+    }
+}

tests/Integration/tests/invalid-guides-xml/input/Index.rst

@@ -0,0 +1,5 @@
+=====
+Test
+=====
+
+This should not render due to invalid guides.xml.

tests/Integration/tests/invalid-guides-xml/input/guides.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<guides xmlns="https://www.phpdoc.org/guides">
+    <theme name="typo3docs" />
+    <project title="Test" />
+</guides>