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/AbilityRegistrarTest.php

@@ -182,7 +182,12 @@ final class AbilityRegistrarTest extends TestCase
         $previous = ini_set('error_log', $log);
         $ability = $this->ability();
         $ability->expects('execute')->andThrow(
-            new WriteFailed('Felder konnten nicht gespeichert werden.', false, 5, 811, new RuntimeException('meta /var/www'))
+            WriteFailed::partial(
+                'Felder konnten nicht gespeichert werden.',
+                new RuntimeException('meta /var/www'),
+                postId: 5,
+                revisionId: 811
+            )
         );
 
         try {