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>

src/Integrations/Divi/Pages/DiviReplaceTextAbility.php

@@ -186,7 +186,7 @@ final class DiviReplaceTextAbility implements Ability
             $divi = $this->loader->load($postId, true);
             $problem = $divi instanceof WP_Error ? $divi : $this->loader->writable($divi);
             if ($problem instanceof WP_Error) {
-                $skipped[] = ['post_id' => $postId, 'reason' => self::reason($problem)];
+                $skipped[] = ['post_id' => $postId, 'reason' => Errors::clientMessage($problem)];
                 continue;
             }
 
@@ -320,7 +320,7 @@ final class DiviReplaceTextAbility implements Ability
         } catch (WriteFailed $e) {
             self::log($postId, $e);
 
-            return ['post_id' => $postId, 'ok' => false, 'error' => self::reason($e->toError())];
+            return ['post_id' => $postId, 'ok' => false, 'error' => Errors::clientMessage($e->toError())];
         } catch (Throwable $e) {
             self::log($postId, $e);
 
@@ -343,7 +343,7 @@ final class DiviReplaceTextAbility implements Ability
         $postId = $input['post_id'];
         $page = $this->writer->page($input);
         if ($page instanceof WP_Error) {
-            return ['post_id' => $postId, 'ok' => false, 'error' => self::reason($page)];
+            return ['post_id' => $postId, 'ok' => false, 'error' => Errors::clientMessage($page)];
         }
 
         $post = $page->post;
@@ -475,14 +475,6 @@ final class DiviReplaceTextAbility implements Ability
             : 'Knoten nicht lesbar (Attribute kein JSON-Objekt) – im Divi-Builder bearbeiten';
     }
 
-    /** The client message of an error (its JSON "message"), without the JSON around it. */
-    private static function reason(WP_Error $error): string
-    {
-        $data = json_decode($error->get_error_message(), true);
-
-        return is_array($data) && is_string($data['message'] ?? null) ? $data['message'] : $error->get_error_message();
-    }
-
     private static function log(int $postId, Throwable $e): void
     {
         Log::failure(sprintf('lit/divi-replace-text on post %d', $postId), $e);