Resolve #3459123 "Add basic tests"

Closes #3459123

tests/modules/page_to_pdf_test/src/Plugin/PdfGenerator/TestPdfGenerator.php

+<?php
+
+namespace Drupal\page_to_pdf_test\Plugin\PdfGenerator;
+
+use Drupal\Core\File\Exception\FileException;
+use Drupal\page_to_pdf\Plugin\PdfGenerator\DoppioPdfGenerator;
+
+/**
+ * Provides a Test PDF Generator Service.
+ *
+ * @PdfGenerator(
+ *   id = "test_pdf",
+ *   label = @Translation("Test PDF"),
+ * )
+ */
+class TestPdfGenerator extends DoppioPdfGenerator {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function generatePdf(string $page_url, string $output_file_uri, array $options = []): void {
+    try {
+      // Instead of calling the actual API, directly save the mock PDF data.
+      $this->saveMockPdfToFile($output_file_uri);
+    }
+    catch (\Exception $e) {
+      $this->logger
+        ->error($this->t("Mock PDF generation failed with exception: '@error_message'.", ["@error_message" => $e->getMessage()]));
+    }
+  }
+
+  /**
+   * Save mock PDF data to the given file.
+   *
+   * @param string $path
+   *   Destination file path.
+   */
+  protected function saveMockPdfToFile(string $path): void {
+    // Mock PDF content.
+    $pdf_data = <<<PDF
+%PDF-1.4
+1 0 obj
+<< /Type /Catalog /Pages 2 0 R >>
+endobj
+2 0 obj
+<< /Type /Pages /Count 1 /Kids [3 0 R] >>
+endobj
+3 0 obj
+<< /Type /Page /Parent 2 0 R /MediaBox [0 0 612 792] /Contents 4 0 R >>
+endobj
+4 0 obj
+<< /Length 55 >>
+stream
+BT
+/F1 24 Tf
+100 700 Td
+(This is a mock PDF file.) Tj
+ET
+endstream
+endobj
+5 0 obj
+<< /Type /Font /Subtype /Type1 /BaseFont /Helvetica >>
+endobj
+xref
+0 6
+0000000000 65535 f 
+0000000010 00000 n 
+0000000074 00000 n 
+0000000123 00000 n 
+0000000220 00000 n 
+0000000321 00000 n 
+trailer
+<< /Size 6 /Root 1 0 R >>
+startxref
+390
+%%EOF
+PDF;
+
+    // Create directory.
+    $directory = $this->fileSystem->dirname($path);
+    if (!$this->fileSystem->prepareDirectory($directory)) {
+      if (!$this->fileSystem->mkdir($directory)) {
+        throw new FileException(\sprintf('Could not create the directory %s.', $directory));
+      }
+    }
+
+    // Save the file.
+    if (file_exists($path) && !is_writable($path)) {
+      throw new FileException(\sprintf('The file %s is not writable.', $path));
+    }
+
+    if (!$this->fileSystem->saveData($pdf_data, $path)) {
+      throw new FileException(\sprintf('The file %s could not be saved.', $path));
+    }
+  }
+
+}