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/WordPress/Menus/MenuSetItemsAbilityTest.php

@@ -242,7 +242,7 @@ final class MenuSetItemsAbilityTest extends TestCase
     {
         $writer = Mockery::mock(MenuWriter::class);
         $writer->allows('plan')->andReturn(['errors' => [], 'removals' => [], 'warnings' => []]);
-        $writer->expects('apply')->andThrow(new WriteFailed('Menü nicht gespeichert.', true));
+        $writer->expects('apply')->andThrow(WriteFailed::rolledBack('Menü nicht gespeichert.'));
         $log = Mockery::mock(ChangeLog::class);
         $log->shouldNotReceive('recordPartial');
         $ability = new MenuSetItemsAbility($writer, new MenuOperations(), $log);
@@ -255,7 +255,7 @@ final class MenuSetItemsAbilityTest extends TestCase
     {
         $writer = Mockery::mock(MenuWriter::class);
         $writer->allows('plan')->andReturn(['errors' => [], 'removals' => [], 'warnings' => []]);
-        $writer->expects('apply')->andThrow(new WriteFailed('Menü nicht gespeichert.', false));
+        $writer->expects('apply')->andThrow(WriteFailed::partial('Menü nicht gespeichert.'));
         $log = Mockery::mock(ChangeLog::class);
         $log->expects('recordPartial')->with('lit/menu-set-items', 'nav_menu', '3', Mockery::type('string'));
         $ability = new MenuSetItemsAbility($writer, new MenuOperations(), $log);