Send extra request headers and throw PreconditionFailedException on 412

src/Id/Exceptions/PreconditionFailedException.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Anikeen\Id\Exceptions;
+
+/**
+ * A conditional write failed (412): the stored version wasn't the one sent with `If-Match`
+ * ("preference_version_mismatch"), or the key was already set and the write was create-only
+ * ("preference_exists"). Reload, merge and retry with currentVersion().
+ */
+class PreconditionFailedException extends ApiException
+{
+    /**
+     * The version stored now, null when the key isn't set (or Anikeen ID didn't send one).
+     */
+    public function currentVersion(): ?int
+    {
+        $version = $this->body()->current_version ?? null;
+
+        return is_numeric($version) ? (int) $version : null;
+    }
+}