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/Integrations/Divi/Pages/DiviReplaceTextAbilityTest.php

@@ -561,7 +561,7 @@ final class DiviReplaceTextAbilityTest extends TestCase
     public function testWriteFailedKeepsItsOwnMessage(): void
     {
         $ability = $this->ability();
-        $this->failures = [new WriteFailed('Die Datenbank hat das Speichern abgelehnt.', true, 7)];
+        $this->failures = [WriteFailed::rolledBack('Die Datenbank hat das Speichern abgelehnt.', postId: 7)];
         $log = (string) tempnam(sys_get_temp_dir(), 'lit-log');
         $previous = ini_set('error_log', $log);
         try {