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/Abilities/ErrorsTest.php

@@ -114,4 +114,24 @@ final class ErrorsTest extends TestCase
         self::assertStringContainsString('Entwurf', $payload['message']);
         self::assertArrayNotHasKey('post_id', $payload);
     }
+
+    public function testClientMessageReadsTheJsonMessageBack(): void
+    {
+        self::assertSame('Titel fehlt.', Errors::clientMessage(Errors::invalidInput('Titel fehlt.')));
+        self::assertSame('WordPress sagt nein', Errors::clientMessage(new \WP_Error('db', 'WordPress sagt nein')));
+        self::assertSame('{"code":"x"}', Errors::clientMessage(new \WP_Error('db', '{"code":"x"}')));
+    }
+
+    public function testCodeOfDropsTheLitPrefixOnly(): void
+    {
+        self::assertSame('validation_failed', Errors::codeOf(Errors::invalidInput('x')));
+        self::assertSame('db_error', Errors::codeOf(new \WP_Error('db_error', 'x')));
+    }
+
+    public function testFieldsOfReturnsTheFieldsOrNothing(): void
+    {
+        self::assertSame(['title' => 'Pflichtfeld'], Errors::fieldsOf(Errors::invalidInput('x', ['title' => 'Pflichtfeld'])));
+        self::assertSame([], Errors::fieldsOf(Errors::invalidInput('x')));
+        self::assertSame([], Errors::fieldsOf(new \WP_Error('db', 'x', 'kein Array')));
+    }
 }