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/Media/MediaUploadAbilitiesTest.php

@@ -108,7 +108,7 @@ final class MediaUploadAbilitiesTest extends TestCase
 
     public function testAPartialUploadIsLoggedAsPartialAndRethrown(): void
     {
-        $failure = new WriteFailed('Alt-Text fehlt.', false, 55);
+        $failure = WriteFailed::partial('Alt-Text fehlt.', postId: 55);
         $importer = Mockery::mock(MediaImporter::class);
         $importer->allows('fromUrl')->andThrow($failure);
         $log = Mockery::mock(ChangeLog::class);