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/DiviWriterTest.php

@@ -171,7 +171,7 @@ final class DiviWriterTest extends TestCase
         $content = DiviPageLoaderTest::fixture('synthetic-basic.html');
         DiviPageLoaderTest::givenPost($content);
         $saver = Mockery::mock(DiviSaver::class);
-        $saver->allows('save')->andThrow(new WriteFailed(DiviSaver::REASON, false, 7, 811));
+        $saver->allows('save')->andThrow(WriteFailed::partial(DiviSaver::REASON, postId: 7, revisionId: 811));
         $log = Mockery::mock(ChangeLog::class);
         $log->expects('recordPartial')->with('lit/divi-delete', 'page', '7', 'Divi "Start": Änderung nur teilweise zurückgenommen', 811);
         $writer = $this->writer($saver, $log);
@@ -187,7 +187,7 @@ final class DiviWriterTest extends TestCase
         $content = DiviPageLoaderTest::fixture('synthetic-basic.html');
         DiviPageLoaderTest::givenPost($content);
         $saver = Mockery::mock(DiviSaver::class);
-        $saver->allows('save')->andThrow(new WriteFailed(DiviSaver::REASON, true, 7, 811));
+        $saver->allows('save')->andThrow(WriteFailed::rolledBack(DiviSaver::REASON, postId: 7, revisionId: 811));
         $log = Mockery::mock(ChangeLog::class);
         $log->expects('recordPartial')->never();
         $log->expects('record')->never();