refactor: PostRow::update and PostGuard::assertNoMissedSchedule (M5)

- PostRow::update(id, columns, failure) / ::save(): the slashed wp_update_post of
  internal row writes, refusals as RuntimeException '<failure>: <why>'. Used by
  ContentUpdate::restoreRow, PostCloner::updateRow, MediaMetaWriter apply/restore.
- PostGuard::assertNoMissedSchedule(postId, message) replaces the re-read + check in
  ContentUpdate::restoreRow, PostCloner and DiviSaver::restore (DiviSaver keeps its
  own kses window; M4).
- PostCloner::restore uses Rollback::attempt.
Log messages unchanged.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/Unit/Integrations/WordPress/PostRowTest.php

@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\WordPress;
+
+use Brain\Monkey\Functions;
+use LIT\MCP\Integrations\WordPress\PostRow;
+use LIT\MCP\Tests\Unit\TestCase;
+use RuntimeException;
+use WP_Error;
+
+final class PostRowTest extends TestCase
+{
+    public function testUpdateSavesTheSlashedColumnsWithTheId(): void
+    {
+        Functions\when('wp_slash')->alias(static fn (array $data): array => $data + ['slashed' => 1]);
+        Functions\expect('wp_update_post')->once()
+            ->with(['post_title' => 'A', 'ID' => 5, 'slashed' => 1], true)
+            ->andReturn(5);
+
+        PostRow::update(5, ['post_title' => 'A'], 'nie');
+    }
+
+    public function testUpdateThrowsTheFailureWithWordPressReason(): void
+    {
+        Functions\when('wp_update_post')->justReturn(new WP_Error('db', 'Datenbank weg'));
+
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('Post 5 could not be updated: Datenbank weg');
+        PostRow::update(5, ['post_title' => 'A'], 'Post 5 could not be updated');
+    }
+
+    public function testSaveReportsAZeroResultAsUnknownError(): void
+    {
+        Functions\when('wp_update_post')->justReturn(0);
+        self::assertSame('unknown error', PostRow::save(5, ['post_title' => 'A']));
+
+        Functions\when('wp_update_post')->justReturn(5);
+        self::assertNull(PostRow::save(5, ['post_title' => 'A']));
+    }
+}