test: AssertsToolErrors helper for tool error assertions (T3)

tests/Support/AssertsToolErrors (used by TestCase): assertToolError($result, code)
checks the WP_Error code lit_<code> and the code of the client JSON and returns the
payload; clientError() decodes the payload; errorFields() returns the fields and
asserts they exist. Replaces the private clientError() decoders of
DiviWriteAbilitiesTest/DiviReplaceTextAbilityTest, 216 single-line
assertSame('lit_…', …->get_error_code()) assertions, 24 get_error_data()['fields']
digs and two json_decode(message) reads. ErrorsTest keeps json_decode: it tests
the format itself. Tests 1404 -> 1409 (AssertsToolErrorsTest); no test has fewer
assertions.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/Unit/Integrations/WordPress/Status/PostUnpublishAbilityTest.php

@@ -52,7 +52,7 @@ final class PostUnpublishAbilityTest extends TestCase
             self::assertFalse($e->rolledBack);
             self::assertSame(5, $e->postId);
             self::assertStringContainsString('save_post-Hook kaputt', $e->getMessage());
-            self::assertSame('lit_partial_write', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'partial_write');
         }
         $log->assertPartials(['ability' => 'lit/post-unpublish', 'object_type' => 'page', 'object_id' => '5']);
     }
@@ -63,7 +63,7 @@ final class PostUnpublishAbilityTest extends TestCase
 
         $result = $this->ability(FakeChangeLog::forbidden(), 'trash')->execute(['post_id' => 5]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testDraftIsANoOp(): void