refactor: WriteFailed named constructors, Rollback runner, Errors read-back helpers (H6)

- WriteFailed::rolledBack()/partial() replace the 11-parameter constructor (now
  private); the hint/state of either error are plain $hint/$state.
- Rollback::attempt(...steps) runs every undo step and collects failure messages;
  used by ContentCreate::discard, ContentUpdate::rollBack, MediaUpdate::rollBack and
  DiviSaver::restore.
- Errors::clientMessage/codeOf/fieldsOf replace DiviReplaceTextAbility::reason,
  the JSON digging in MediaUpdateAbility::failure and ContentCreateAbility::fields.
- Errors docblock documents the exception/WP_Error split.
- ContentCreateAbility::discard takes the client reason as a parameter (both callers
  still pass REASON_EXTRAS; changing the Divi text needs a ruling).

Error payloads unchanged.

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

tests/Unit/Abilities/RollbackTest.php

@@ -0,0 +1,54 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Abilities;
+
+use LIT\MCP\Abilities\Rollback;
+use LIT\MCP\Tests\Unit\TestCase;
+use RuntimeException;
+
+final class RollbackTest extends TestCase
+{
+    public function testAllStepsRunInOrderAndNothingFailed(): void
+    {
+        $ran = [];
+
+        $failures = Rollback::attempt(
+            static function () use (&$ran): void {
+                $ran[] = 1;
+            },
+            static function () use (&$ran): void {
+                $ran[] = 2;
+            }
+        );
+
+        self::assertSame([], $failures);
+        self::assertSame([1, 2], $ran);
+    }
+
+    public function testAFailingStepDoesNotStopTheOthersAndItsMessageIsCollected(): void
+    {
+        $ran = [];
+
+        $failures = Rollback::attempt(
+            static function (): void {
+                throw new RuntimeException('Begriffe nicht gelöscht');
+            },
+            static function () use (&$ran): void {
+                $ran[] = 2;
+            },
+            static function (): void {
+                throw new \Error('Zeile nicht zurückgesetzt');
+            }
+        );
+
+        self::assertSame(['Begriffe nicht gelöscht', 'Zeile nicht zurückgesetzt'], $failures);
+        self::assertSame([2], $ran);
+    }
+
+    public function testAReturnValueIsNoFailure(): void
+    {
+        self::assertSame([], Rollback::attempt(static fn (): string => 'egal', static fn (): bool => false));
+    }
+}