fix: answer null instead of 500 when the export marker is missing

src/BusProNet/Model/XmlExportInfo.php

@@ -4,6 +4,9 @@ declare(strict_types=1);
 
 namespace App\BusProNet\Model;
 
+use League\Flysystem\FilesystemException;
+use League\Flysystem\FilesystemOperator;
+
 final readonly class XmlExportInfo
 {
     private const INFO_FILENAME = 'uebertragung.info';
@@ -20,6 +23,26 @@ final readonly class XmlExportInfo
         return self::INFO_FILENAME;
     }
 
+    /**
+     * Reads the marker from a storage, or null when it is missing or unreadable.
+     *
+     * A marker caught mid-sync or truncated is treated as absent rather than as an error:
+     * callers only ever want to know which export they are looking at, and "unknown" is
+     * an answer they can act on.
+     */
+    public static function fromStorage(FilesystemOperator $storage): ?self
+    {
+        try {
+            if (false === $storage->fileExists(self::INFO_FILENAME)) {
+                return null;
+            }
+
+            return self::fromString($storage->read(self::INFO_FILENAME));
+        } catch (FilesystemException|\InvalidArgumentException) {
+            return null;
+        }
+    }
+
     public static function fromString(string $content): self
     {
         $content = mb_convert_encoding($content, 'UTF-8', 'Windows-1252');